The Application of Photography to the Microscope [by M. H. Stiles]
I propose tonight to describe to you & to illustrate experimentally how Photographs may be taken with the microscope, more especially with the lower power & with simple & inexpensive apparatus.
The application of Photography to the microscope dates back many years even to the old Daguerreotype process.
Until recently wet plate were invariably employed, but considerable inconvenience attended their use especially with high powers where long exposures were necessary; but since the introduction of Gelatine dry plates the process of Photography has been rendered so much more simple that its details are readily mastered & at the same time the results are to my mind decidedly superior.
The most available & most manageable source of Light for Photo-micrography (to say nothing of cheapness) is the ordinary Paraffin Lamp used by the microscopist. Those who have time & means at their disposal may employ either daylight, the electric light, the Lime Light or the magnesium Light, but these are expensive & beyond the reach of the ordinary amateur & as good results can be obtained with the light from a paraffin lamp I shall treat only of that in this paper. The subject naturally divides itself into three stages – the arrangement of the apparatus – taking the negative – and printing the positive.
The apparatus consists of the microscope, an ordinary camera with the lens removed, & a lamp as a source of light. For convenience in working, it is well to have these arranged on a baseboard with a stage at one end, upon which the camera may be clamped in such a position that the centre of the lens mount shall exactly agree with the axis of the microscope when placed horizontally.
To readily find the correct position for the Instrument I have fixed three thin pieces of wood to receive the feet of then tripod, these are just the width of the tripod and about two inches long so as to allow a little motion in a longitudinal direction.
The mirror of the microscope is removed. The lamp is raised on a stage at such a height that the centre of the flame, when giving its best light, is in the axis of the microscope.
Between the lamp & the object to be photographed the Bull’s Eye Condenser is placed with its flat side to the object, the condenser being about 2 ½ to 3 inches from the object & about 2 inches from the flame, these distances must of course be varied to suit different requirements; A bright but uniform light should be used. With powers below the one inch the flat flame may be used; with the 1 inch I am accustomed to use an oblique flame; with the ½ inch & 1/5 inch the edge of the flame & a Webster Condenser are employed. Photographs may be taken with or without the eyepiece, by the latter methos a camera extending up to 36 inches is required. With the eyepiece a much shorter one may be used & the results are equally good if not better whilst the shorter arrangement is certainly much more convenient. The focussing screen is of the very finest ground glass, this must be carefully looked so as ordinary ground glass is not suitable.
Microscopic objects not being specially constructed for Photographic work do not, as a rule, give the best results at their ordinary visual focus, in other words, their chemical & visual focus are not usually coincidental. This statement applies more particularly to the lower power & these require a varying amount of correction. This correction is made by slightly withdrawing the objective from the object & the amount of correction must be determined for each objective & when found carefully noted. This may thus be done.
A perfectly flat object such as a carefully cut & mounted transvers section of wood of uniform structure is placed in position & accurately focussed on the ground glass, the image being examined with a lower power lens. The focussing glass is then removed & the dark slide containing a prepared plate is substituted, the front part of the dark slide contains a thin screen which shields ¾ of the plate, the exposed portion being of triangular form with its apex in the centre. Four exposures may then be given on one plate each with a varying amount of correction & what is also important each with a varying length of exposure. The result will show two things the correction required for the production of the sharpest image & the length of time required for exposure, the latter will be merely approximate as different objects, even with the same lens, require different exposures but still it will be a useful guide. The microscope I employed is a Binocular, from this the prism is removed & a stop substituted which served as a shutter which is quicker in its action than the shutter of the dark slide & enables me to control the exposure more easily.
It is important that the whole arrangement should be firm & steady.
TAKING THE NEGATIVE
The Gelatine Dry Plates which up to the present I have used, & which I can certainly recommend as being of excellent quality, are those made by Wratten & Wainwright. For the 1 inch & lower power I use the ordinary plates & for the higher power the Instantaneous ones. The 4 inch objective require ¾ th of a turn of the milled head for correction & an average exposure of 2-3 minutes.
The 2 inch, 3/8 of a turn with an exposure of from 3 to 4 minutes according to the density of the object.
The I inch, 3/16 of a turn & an exposure of 4 to 5 minutes.
The ½ inch, 1/24 of a turn & an exposure of10 to 15 minutes.
A Certain amount of variation in exposure must be allowed according to the distance of the plate from the eyepiece as the size of the image increases & the intensity of the light decreases rapidly on lengthening the camera. My instrument with the No 1 eyepiece & the camera closed gives a disc about 3 inches in diameter, with the camera extended the disc is increased to 4 ½ inches.
After the requisite exposure the plate is ready for development & this development must be performed under a ruby light. A convenient wat of obtaining this is by burning a good hard candle in a holder fitted with a ruby glass chimney & even then it is well to shield the plate when not actually engaged in watching the development.
It must be borne in mind that these gelatine plates especially the instantaneous ones are exceedingly sensitive to light & it is not all ruby glass that can be trusted. A better medium is paper coated with a varnish tinted with magenta & aurine. Still the chimney is more convenient & with care in partially shielding the plate during development will answer the purpose.
The developing, fixing, etc., are conducted in a dish, preferably of ebonite (a shallow tin tray with sloping sides coated with black Japan varnish & well baked forms a cheap & excellent substitute), this should be about ¼ larger each way than the plate. For the ¼ plate size I use the following developer which is a modification of Wratten & Wainwrights.
AB – Developer ordinary. Lig am Fort 3ii Pot Brown 15 gms water 5 oz.
P – Pyrogallic acid 2 grams, water 1 ¼ oz
P – This is mixed in a measure just as required & poured on the plate in the tray the gelatine side being upward.
Whilst this is soaking measure two drams of the AB solution in the same measure as the pyrogallic acid was dissolved, pour the solution from the tray into the measure & then the mixture of the two back on to the plate. With properly timed exposures the image will begin to appear in 20 or 30 seconds & will gradually strengthen until full printing density is obtained. As a rule an intensifier will not be required.
For the instantaneous plates use double the quantity of pyrogallic acid & start the development with ½ dram of A.B. adding the remaining 1 ½ drams when the high lights are fairly out. When the whole of the detail has been fully brought out wash well & immerse in the following fixing solution.
Hyposulphite of soda – 4 oz
Water – 2 oz
It should remain in this for about 10 or 12 minutes or for 5 minutes after the unaltered Bromide of silver has been removed. Pyro development if carried too far is very liable to produce a stained film, this may to a great extent be counteracted by adding neutralized sulphite of sodium in the proportion of 4 grams to each grain of Pyro. The Sulphite should be dissolved in water & neutralized with citric acid adding to the solution of Pyro.
After removal from the fixing solution very thorough washing is necessary in order to get rid of every trace of Hypo. The most rational method of doing this is to place it in a vessel of water near the surface with the face downwards., frequently changing the water (placing the plate in a tray with the face upwards is the worst possible method & yet it is very frequently done). An ordinary basin of such a size that the corners of the plate rest inside near the top is as good as anything for the purpose.
After well washing the plate should be placed for about 5 minutes in a saturated solution of alim (?) containing 5 per cent of Hydrochloric acid. This will both clean & harden the film. Again, wash well & then set up on blotting paper to drain & dry.
With correct exposure full printing density may be obtained, but where this is not the case the image can be intensified as follows.
Three solutions are required
No 1
Sulphate of Iron – 120 grs;
water 8 oz.
Dissolve then add to it a mixture of
Gelatine – 15 grains
Glacial acetic acid – 3 drams
water – 5 drams
This should be made 2 to 3 weeks before it is used.
No 2
Nitrate of silver – 10 grams
Glacial acetic acid – 10 minimal
Water – 1 oz
No 3
Iodine – 2 grains
Iodide of Potassium – 4 grains
Water – 4 oz
Place the washed plate in a clean porcelain dish & pour over it 1 ½ oz of water containing 3 drops of No 3, let it remain in this for one minute, then wash & apply 1 ½ oz Iron solution, pour this back into a measure containing one dram of No 2 & then again on to the plate & keep the dish in motion until sufficient density is obtained.
They may also be intensified by soaking in a solution of Bichloride of mercury (1-60) until the film is well whitened, washing thoroughly & then in a weak solution of ammonia until it becomes black & te required density. When dry the negative should be warmed & varnished.
PRINTING
Prepared sensitized paper may now be obtained of such excellent & uniform quality that it is certainly cheaper & better for the amateur to buy it than to attempt to sensitize it.
Print two or three shades deeper than the finished picture is required & soak in three waters before toning or until the washing water ceases to be milky.
TONING BATH
Chloride of Golds – 4 grams
Acetate of Soda – 2 drams
Distilled water – 1 pint
This should be made 24 hours before being used & should be kept in the dark.
When the prints have acquired the desired tint remove them from the bath & wash slightly before fixing.
FIXING BATH
Hyposulphite of Soda – 2 oz
Water – 1 pint
The prints should remain in this for 10 to 15 minutes moving frequently. They should then be well washed in several changes of water, finishing in tepid water. If this washing be thoroughly performed the prints will keep their colour. Fading is invariably due to imperfect washing. Indeed, the golden rule throughout the whole process of photography is “Wash Well”
There is one branch of Photomicrography which is exceedingly useful & attended with very pleasing results. I refer to the production of Transparent Positives on Glass for exhibiting with the Lantern. An ordinary dry plate 3 ¼ inches square is exposed in contact with a good sharp negative in a printing frame, the usual one used for paper prints answers well if the springs are slightly weakened. With a small Paraffin Lamp placed at a distance of 20 inches from the printing frame, an exposure of 60 seconds is required, The Ferrous oxalate developer should be employed as this produces clean plates & a more pleasing tone than Pyro, it is also more manageable. The following has given good results.
FERROUS OXALATE DEVELOPER
No 1
Sulphate of Iron – 1 oz
Acid Tart – 20 grs
Distilled water – 4 oz
Dissolve & label 1
No 2
Oxalate of Potash (neutral) – 4 oz
Water distilled to – 16 oz
Dissolve & mix
At the time of using, add 3 drams of No 1 to 9 drams of No 2, this is sufficient for a plate 3 1/3 square; the same developer will serve for several plates, before the developer is poured on, the plate should be soaked for a minute or so in distilled water & after development if should be soaked in three changes of distilled water, this method will to a great extent obviate the formation of an opalescent scum on the plate, which is invariably met with when common water containing lime is used.
The process of fixing is the same as first described. To get pictures having a nice tone first obtain a rather thin positive with plenty of detail & then intensify with the mercury & ammonia process taking care to was thoroughly. A very pleasing result will thus be obtained.
The alum solution with 5 per cent of H Cl is employed as usual to harden & clear the film before Intensification.
May 3, 1882.