Photographic Journal 1916

To mount the bull’s-eye a circular hole rather larger than the lens, including the mount, is cut in the upright. The centre of the hole must be opposite the optical centre of the microscope. The bull’s-eye is mounted on an oblong panel sufficiently large to cover the hole in the upright support and to allow for a fixing device. The question may be asked, why not have the hole in the support the correct size for the bull’s-eye and accurately centred once for all? Firstly, there is the difficulty of absolute accuracy. Secondly, you will find that if the bull’s-eye is centred for one objective it is probably wrong for another; thirdly, that the addition of a polarizer to the substage condenser without any alteration in the microscope will upset the adjustment. Here is (fig. 3) the simple method I use for adjusting the bull’s-eye – really a short focus portrait lens, as I have mentioned earlier. First, of course, a large hole is made in the centre to take the flange or other mounting of the lens. Holes about an inch in diameter are made with a centre bit in each corner of the panel. Two strips of wood grip the panel against the upright by four screws which go through the centres of the holes in the panel. It is easy to adjust the screws so that the panel is held tightly enough to resist the law of gravity but loose enough to be shifted by hand for centring the bull’s-eye with perfect accuracy.

Having described the stand, I will now place the microscope in position. The microscope should be placed so that its central axis is parallel to and directly over the central line of the base board. Once this position is found, blocks of wood should be screwed down between the feet or salient points of the microscope so that at any time it can be put instantly in the correct position against these guide blocks. You will notice the support for the tube of the microscope in use. The distance between the microscope and the bull’s-eye is regulated by circumstances. With a large light source, such as an incandescent gas mantle or the flat flame of a paraffin lamp, the separation need not be great, in this case the distance from the diaphragm of the bull’s-eye or paralleliser to the substage iris is 7¼ inches, or from the mantle to the latter 13 inches. Any colour filter which may be necessary can be put in a home-made carrier fitting the substage as shown here.

Underneath the staging supporting the camera runs a quarter-inch steel rod which is connected to the fine adjustment of the camera by an endless band of crochet chain. If the length of the band is right the system works quite well: it is unnecessary to fix a pulley on to the rod. This arrangement gives an extra fine adjustment and can be used with one’s head under the focussing cloth.

A rough and ready way of making a light-tight junction between microscope and camera is to employ a padded ring of stout card or thin wood. The hole in the ring fits the tube of the microscope: there is generally a convenient shoulder near the eyepiece which will make a bearing for it, whilst the outer circle of the ring should be large enough to overlap the flange in the camera front.

Having described the fixing up of a photomicrographic camera, we may now consider the optical arrangements. For a 2-inch objective the bull’s-eye lens alone may be used to light a transparent object, but for an inch and higher powers an achromatic condenser should be placed in the substage. One advantage of the bijou gas mantle as a light source is the ease with which even illumination can be obtained. There is no difficulty in filling a large condenser with light, as even with a slight enlargement of the image of the mantle the opening in the substage iris is amply covered. An achromatic condenser should be used, the cheaper chromatic forms are not desirable.

The steps to be taken in making a photomicrograph of low magnification, using the substage condenser, an inch or shorter objective, with an eye piece, are as follows. The object is eliminated and then focused in the microscope by the coarse adjustment, then the slide is shifted a trifle to get the object out of the field. Next with a large aperture in the substage condenser the iris or opening of the bull’s-eye is focused in the field of the microscope by means of the rack and pinion or other similar adjustment of the substage. The diaphragm or aperture of the bull’s-eye may be regarded as the light source, and its critical or sharp image is now focused on the object. An iris diaphragm to the bull’s-eye is a great convenience: one may go further and say that it is essential to a proper centring of the light if a number of objectives are used alternately. The next thing is to centre the lighting system, beginning with the substage condenser. The substage iris is closed then, with the eye piece mounted, looking down the tube of the microscope the opening in the substage iris will be seen through the objective. The opening should appear in the centre of the objective, if not the position of the substage iris is altered by the centring screws to the substage until the opening is concentric with the objective. If there are no centring screws the best position may be obtained by turning the condenser in the sleeve of the substage. An alternative method is to focus the substage iris by racking up the microscope with the eyepiece in position and centre as before; this is a better plan than racking down the substage. The best means I know of requires a centring eye-glass, a fitting formerly supplied with high-class instruments. This appliance fits over the eyepiece, from which the cap has been removed. There are two plano-convex lenses set in the top tube and the cap has a small central hole. The lens tube slides for focusing purposes in a plain tube which fits over the eyepiece of the microscope. With this instrument it is easy to focus the apertures of the objective and the substage iris and to adjust them without disturbing the focus of either.

After centring the condenser the adjustment of the bull’s-eye is taken in hand; this is done by closing its iris diaphragm and moving the adjustable panel until the aperture is central when viewed through the microscope. Finally a sharp image of the mantle should be seen on the substage iris: if that image is out of focus it can be remedied by sliding the tray bearing the burner in or out. The position of the mantle is altered by sliding the burner up or down on the pin or by slewing it round, so that the diaphragm aperture is in the centre of the image of the mantle.

Sometimes with lenses of large aperture the bull’s-eye iris or opening, though sharp looks ghostly: this is due to spherical aberration in the lighting system. To correct this defect the substage iris should be closed until the foggy appearance gives way to a strong image.

Now all is ready for properly viewing the object, and the selected portion is arranged in position. The camera is placed on its support with the lens flange pushed home against the padded ring on the microscope and the object is again focussed on the screen. For smaller or larger images the camera is racked in or out and re-focussed.

The exposure, as in ordinary enlarging, requires to be increased directly with the square of the magnification. A photomicrograph of 25 diameters requires, other things being equal, one-quarter the exposure necessary for one of 50 diameters. The actual exposure required depends further upon the aperture of the objective and the nature of the object. A thin section or the skin of a fruit free from red colour, with the lighting described and a magnification of 100 diameters, will take 3 to 20 minutes with a K111 screen and isochromatic plate.

The amount of magnification employed can be ascertained by means of a stage micrometer, which is a microscope slide on which are fine lines ruled 1/100 and 1/1000 of an inch apart. Where a French measure is available, a micrometer divided to 1/10 and 1/100 of a millimetre is more convenient. The micrometer is placed on the stage of the microscope and its engraved scale focussed on the camera screen by the focussing pinion of the microscope. The resulting increase in the size of the scale gives the magnification. It is awkward to compare the scale with an ordinary measure placed against the camera screen; a carefully notched card is easy to work with. Supposing that the micrometer is marked with 1/100 of an inch, then if four of these spaces take up an inch the magnification would be 25 diameters. It saves a considerable amount of time and trouble to work out a table of the magnifications obtained with one’s objectives and eye pieces at different camera extensions. For low-power lenses, 2 inches or 1 inch, the table may advance 5 diameters at each step; for half-inch objectives 10 or 25 diameters might be the basis. There is seldom any need to work at intermediate magnifications such as, let us say, 34 diameters with a 2-inch objective, and I hold it would be better to work at either 30 or 35 diameters in that case. In practice the object is arranged upon the camera screen at about the scale required, the camera is set to the nearest measurement given in the table for the particular objective and eye piece, and the object is then finally focussed.

A thorough knowledge of the capabilities of various methods of illumination is valuable; some objects are seen best in one way, some in another. Next to transmitted light or axial illumination comes dark ground illumination. The method consists of cutting off the access of all direct light to the objective, lighting the object with oblique rays which cannot enter the objective direct. The object scatters and reflects light, and this light entering the objective reveals the object brightly lit against a background. There are several appliances for this work; the spot lens and the old paraboloid are useful for low-power work. I have not had much experience with the spot lens but prefer the paraboloid for a ½ or 1-inch lens to the modern condenser with a central spot as it gives me a darker background. I have here the paraboloid I refer to – it works well with 12 mm. apochromat of large aperture.

In arranging the light for dark ground illumination we proceed as for transmitted light and when all is in order a central stop is put into the condenser, the iris diaphragm opened out to its full extent. The size of the central stop should be regulated by the aperture of the objective. A measure of the size required can be obtained by setting the iris diaphragm to the aperture of the objective in use ; a suitable stop will then cut off all light from the microscope without overlapping the lens opening appreciably. With an expanding stop the lever can be moved until no light enters the lens and the iris diaphragm opened out then.

Dark ground illumination will often show clearly detail that is invisible by transmitted light; it shows diatoms and polycystina effectively and cellular tissue of plants and fruits.

The polariscope, which is more often regarded perhaps as a pretty scientific toy rather than an eminently useful adjunct to the microscope, also gives dark ground effects with some objects. It is useless for the examination of diatoms but is valuable in analytical work. It is rather surprising that very little information is given about the polariscope in text-books. It should be used in conjunction with the substage condenser, the Nicol prism (the polariser) being placed as near as possible to the sub-stage iris. With the polariser added, the light and optical train are centred as already described for axial illumination. If the analyser, another Nicol prism, screws between the objective and the nose piece of the microscope it should be in position whilst the adjusting is done. If the analyser fits over the eye piece, on the whole a preferable arrangement, it need not be put on before the lighting system is in order.

If a selenite is used the best position for it is at the back of the condenser; the colouration is more even than it is likely to be if a selenite in slide form is used upon the stage. Besides, in the latter case it will often be impossible to focus the condenser through the extra piece of glass. A selenite mounted between two ¾ -inch cover glasses can be bought for less than a shilling; it is then mounted in a flat cardboard or metal ring made to fit the stop carrier of the condenser. In what direction a selenite is useful I am not learned enough to say, but it offers much inducement to wasting time and the exposure of colour plates.

The polariscope is of great service in food examination for discovering adulteration in butter and jams and admixtures of starch grains. It will bring into prominence details of plant structure that otherwise are imperfectly seen.

The Lieberkiihn, a concave metallic mirror fitting over the lens, is especially useful for examining objects in their natural state such as parts of plants and living insects. The substage condenser is removed and the light focused by the bull’s-eye up on the little mirror, which in turn focusses it upon the object. Generally it is necessary to cut off light behind the object by a dark cell. With ordinary microscopic slides this method of lighting gives a dark ground effect similar to that obtained with a paraboloid or condenser with central stop, but its use is only practicable with low-power lenses.

I have omitted so far referring to the actual photographic part of the work, because hints on the microscopic side would seem to be more generally useful to members of this society. But before concluding, I should like to say something about colour screens and plates. Photomicrography differs from the ordinary run of camera work: generally speaking we aim at giving a correct representation of an object with the aid of the camera, using isochromatic plates, or better still, panchromatic plates with one form or another of colour filter. With the microscope, correct colour rendering may be and often is a secondary consideration. Some particular detail may have to be accentuated; if this can be done by false colour rendering, no scruples will stand in the way. The greater assortment of colour screens than a used by the ordinary photographer is very desirable in order that an object may be photographed by the light it transmits forgetting the maximum detail, or by its complementary colour for enhancing contrast. Of course, in order to photograph with screens of fancy colours, panchromatic plates are necessary. Isochromatic plates are generally preferable to ordinary plates as being more sensitive to artificial lights. For information upon the subject of colour screens, the pamphlet “Photomicrography,” published at threepence by the Wratten Division of Kodak, Ltd., is useful.

Over and above the question of colour rendering, a colour screen or screens are valuable for improving the definition of imperfectly corrected lenses; with suitable light filters a good achromatic objective will vie in performance with far more costly apochromats, whilst for high-power photomicrography, even with the finest lenses, filters are necessary for the best results.

Mr. W. T. P. CUNNINGHAM said he had enjoyed Mr. Marriage’s demonstration very much. The lecturer had shown what fine work could be done with a simple home-made apparatus. He (the speaker) only used a home-made apparatus himself, and he could appreciate the details Mr. Marriage had brought before them. He would like to ask Mr. Marriage a question with regard to lenses. What outfit of lenses did he recommend—how many, and of what power—and also the form of eyepiece, for a beginner to start with? He did not mean an expensive outfit, but an outfit which would give a beginner a satisfactory plant wherewith to do a certain amount of work.

Mr. MARRIAGE said, speaking from his own personal experience, that he found himself using more than anything else a 3.5 lens. A beginner really could do the bulk of the work he required with a 1-inch lens, and he would not counsel anyone at the outset to buy anything else than a 1-inch or something approaching that in focal length. As to the eyepiece, he would suggest one eyepiece of, say, twice magnifying power, and another of perhaps six times. He would not recommend the beginner to lay out money on other lenses until he had thoroughly mastered the 1 inch. He would gain experience with this lens, and after a time he would be better able to judge what further expenditure it was worth while for him to incur. A very good 1-inch lens could be got for something under £1. As for the eye piece, there was no particular advantage in going to a high price in getting compensating oculars for these low-power lenses. The ordinary Huyghenian ocular was quite sufficient, he thought. Certainly the condenser ought to be an achromatic one. There was another advantage, of course, in using low-power objectives, namely, that one could, if one pleased, use them without any eye piece at all. Some people preferred to use lenses without any eye piece. Largely it was a question of convenience whether one chose to get the magnification with an eye piece, or without an eye piece, by means of greater camera extension.

The CHAIRMAN asked whether there was any advantage in working with the eyepiece or without the eye piece. What was the reason for sometimes using it, and at other times not? In such a subject as jam, for example, when one was looking for apple starch grains, would one use an eye piece or not? Then with regard to the exposure, could Mr. Marriage give them any idea of the necessary exposure. Was it entirely found out by trial and error, and if so, how did one proceed? Did one work on different parts of each plate, or on different plates for each experiment?

Mr. MARRIAGE said, in reply, that with regard to the question of using the objective alone as compared with the use of an objective in conjunction with an eye piece, he preferred to use and objective with the eyepiece rather than to use the objective alone. It was more a question of convenience than anything else. If one wished to obtain a high magnification without an eye piece, then one required considerable camera extension and had to lengthen the whole apparatus considerably, and difficulties came in with the tube of the microscope. If he were going to work with his microscope without an eye piece, he would remove the tube of the microscope and then bring the camera closer up.

The CHAIRMAN asked whether Mr. Marriage had any trouble as regarded the chemical focus and the visual focus. Was any difference in position made on this account?

Mr MARRIAGE said that he had done a considerable amount of work with first-class lenses dating from the middle of the last century – about 1858 – and he found them well corrected. The difficulty involved could be overcome to a large extent, if not entirely done away with, by using colour screens for one region of the spectrum – not exactly monochromatic. As to exposure, he used the trial and error method, of course keeping notes of exposures made in the past as a guide for the future. Probably the best way to arrive at the correct exposure was to make strip exposures across the negative. Dr. Duncan Reid read a paper before the Society some time ago on a method of standardizing exposures, and the particulars he gave on that occasion were valuable.

Mr. PARFITT asked whether Mr. Marriage focussed on the ground glass or on the clear part.

Mr. MARRIAGE said that he did both. He got the rough focus on the ground glass, and if this was not sufficient he put the focussing magnifier on to the clear part, which was, of course, a cover glass cemented down on to the ground glass. But he found that, for low-power work at any rate, he could do almost as well by observing on the ground glass. Another method was to have a thin plate-glass screen behind and work only with the focussing magnifier. Yet another method, which was shown by Dr. Reid on the occasion just referred to, was to have a 7-inch focus lens focussed on the air-space only, and to use no ground glass whatever.

The CHAIRMAN asked how in this case he hit the exact plane.

Mr. MARRIAGE said that he thought Dr. Reid had a block which fixed the correct distance from the end of the camera. It was a very beautiful way of focussing the image, and one got much more illumination, of course, than one had in any ground glass.

Mr. J. MclNTOSH said that he thought one way of getting over the accommodation trouble was by moving the head slightly from side topside. If the image altered, it proved that the planes were not in focus.

Mr. MARRIAGE, in reply to a further question as to whether an ordinary projection lantern lens would answer as well as his paralleliser, said yes, but generally the former is a lens of 6-inch equivalent focus, which increases the necessary distance between the light and the microscope.

Mr. CUNNINGHAM said that Mr. Marriage had told them that he focussed the light of the mantle on to his substage condenser iris, and then focussed the image of the iris on to his object. By that means it seemed to him (the speaker) that one would have the mesh of the mantle showing very badly on the ground glass of the camera. Did he use any diffusing medium or screen to overcome that, or was it negligible?

Mr. MARRIAGE said that he never used a diffusing screen, and he did not meet with the difficulty Mr Cunningham had suggested. The mesh did not appear on the ground glass. If it did appear he would alter the position of the light. Generally if the light was focussed on the iris diaphragm of his substage condenser, it did not show on the ground glass. He tried, when first constructing the apparatus, to arrange for a ground glass behind the condenser, but he had so much trouble from the glass cracking that he gave it up. The ground glass near to the light always cracked. Even when there was no grip upon it at all, it would crack.

Rev. F.C.LAMBERT said that he would like to ask Mr. Marriage one question. He was not quite clear upon the focussing eye piece he used, which was practically a pinhole. Was he right in supposing that it was merely an aid to localisation of the particular part one wanted to photograph?

Mr. MARRIAGE said that the idea of the focussing eyepiece was to see when the diaphragm aperture of the substage condenser was central. It was a centralising device.

Mr. LAMBERT showed by demonstration with Mr. Marriage’s instrument his own device, which consisted of a cardboard tube with a hole in it, sliding backwards and forwards at the end of the Huyghenian eye piece, so that the observer was compelled to look straight. Its purpose was to prevent the eye from wandering over the eye lens of the eye piece.

Mr. MARRIAGE said that with his arrangement one did not have to shift the apparatus. Its advantage was that without altering any of the adjustments, he could see the iris diaphragm.

Mr. LAMBERT proceeded to suggest one or two other points. With regard to the focussing screen, it was a very important thing to have a fine grain focussing screen. He had tried a number of screens, and none of them proved so satisfactory as one which was the result of taking an ordinary plate, exposing it very slightly in the dark room by striking a match 3 or 4 feet away and burning it for a second or two—developing, washing, and fixing, and then converting the silver image into a silver iodide; and from this piece of glass, which was uniformly covered, one scraped away a large cross extending from the centre to the sides in horizontal and vertical lines, perhaps ¼ inch or so in width. This enabled one to use the focussing eye piece, and one could perceive at once the edge of the gelatine film, so that one kept his eye adjustment always true to the plane of the ground glass. He had used a small lantern condenser in place of the parallelizer, but this piece of apparatus was not fitted with an iris diaphragm. He would like to know whether it was possible to have an iris diaphragm fitted on to that condenser at a small cost. With regard to the diffusing medium between the gas mantle and the substage condenser, he had used finely ground glass and never had any trouble from breakages. He used a Howellite burner, which he found extremely good, the mantles were particularly durable, not breaking nearly so easily as the ordinary mantles. Before the mantles were burned, they looked like an unshapely rag, but after burning they took a regular formation and gave the best light he had been able to get from gas. The burner was enclosed in something that resembled a coffee canister with a hole in it about ½ inch in diameter, and the diffusing screen was placed perhaps 2 inches or inches from the mantle. He would like to add, with regard to the vexed question of objective versus eye piece, that he certainly did think he got the best results without an eye piece.

Mr. MclNTOSH said that the Howellite burners and mantles had been used in the Society’s studio, and he had had some difficulty in getting them recently. Some discussion took place as to where these mantles could be obtained, Mr. Lambert stating that he had got some about a year ago at a place opposite the Palace Yard, Westminster; and Mr. Cunningham that he had seen them advertised only two or days previously in the Farringdon Road. Mr. Mclntosh, referring to the introduction of the mirror below the mantle, said that obviously the apparent source of light was just as far behind the mirror as it was above it, and it became difficult to get the apparent source of light near enough to the condenser. Another serious objection was that in a very short time – it depended how often one used the thing – the heat from the mantle would, if not crack the glass, burn up the silver behind it. In practice this arrangement was not desirable. He then referred to the question of using, or of not using, an eye piece. Of course, they knew that many objectives ought to be used at a certain distance from the eye piece, and if one did not use an eye piece, and enlarged to various diameters, clearly that distance must alter greatly, whereas the distance between the objective and the eye piece would not alter. Dr. Rodman, he believed, favoured the use of an ordinary eye piece, though he did not know whether Dr. Rodman gave the above reason for using it; at any rate, that reason had been given.

Mr MARRIAGE said that, of course, when one came to use apochromats, then one was bound up with the eye piece in order to get proper achromatism.

Mr CUNNINGHAM said that with regard to the ground glass breaking, he used an inverted incandescent burner surrounded by a black tin screen, and actually in contact with the screen he used a small strip of ground glass. He had had that in use for two years and it had never broken. His aperture was ordinarily 1 inch, but when using a fairly high-power lens, say 1/16th, he found a much better result if he put in a ferrotype stop having an aperture of ¼ inch. There was a little more difficulty in filling the condenser with light, of course, but it was well worth the extra trouble owing to the improved definition. If one got the light badly out of focus, one got the mantle out of focus, too, and it did not show; but this obviously impaired the working efficiency.

Mr. MARRIAGE asked how much Mr. Cunningham’s exposures were increased by the introduction of the ground glass.

Mr. CUNNINGHAM said that the increase of exposure was very much less than he had expected. The ground glass was exceedingly fine, and if he put the very slightest trace of grease upon it, it became very transparent. He should say the increase of exposure was 1¼ to 1½ times ; that would be the maximum increase.

Mr. S. H. WRATTEN asked whether, when starting with the photomicrographic apparatus for the first time, it would not be advisable to have a camera which focussed from the back instead of from the front — or was this of no consequence ?

Mr. MARRIAGE said that he did not think it mattered very much. His camera focussed from the back, but the focussing, in any case, was not done with the camera; the microscope adjustment arranged the focus, and the camera extension was set in order to get the magnification. Asked by Mr. Wratten what happened if, during working, one changed one’s mind with regard to the magnification, Mr. Marriage said that in that case he simply shifted the camera. He never clamped his camera.

Mr. WRATTEN thought the non-clamping of the camera remarkable. In view of possible troubles arising from dampness or dryness of rooms, a little “give” in the camera when inserting the dark slide might readily disturb the calculations. H always clamped each side of his camera, and his adjustments, having once been made, they were finished with.

Mr. MARRIAGE, in reply to Mr. Wratten’s last remarks, said that he could quite see that it might be necessary to clamp certain cameras.

Mr. LAMBERT said that the focussing arrangement was extremely useful. He used a mirror which enabled him to see the focussing glass and to manipulate the various parts of the apparatus. He proceeded to focus by means of the mirror, and looked at it by means of the eye piece on the ground glass, but everything was done by the mirror without the worker having to stoop or cover his head.

The CHAIRMAN Said that nothing remained for him but to express the thanks of the Meeting to Mr. Marriage, who was a past master in all the difficult corners of photography Several of their meetings in recent years had been a little too formal. and not as they were in his own early days, when the people who knew anything told what they knew, and the people who did not know anything, asked questions and learned a great deal. Thanks to Mr marriage, they had had on that occasion the good old-fashioned photographic evening.

The vote of thanks was accorded by acclamation.