The camera is now brought into connection with the microscope, by means of the velvet hood, and racked out into focus at the desired magnification. A hand focussing glass will be a great help in getting exact focus. Of course the ordinary ground-glass focussing screen will not be fine enough to show detail ; a thin cover glass is usually cemented on to the centre with Canada balsam. If the lighting is too brilliant the Davis shutter, a kind of iris diaphragm, may be introduced into the body tube, and indeed is indispensable for many varieties of objects.
Possibly, when the required magnification is reached on the focussing screen the image will be found lacking in sharpness. At the same time, the distance from the back of the camera to the microscope is too great, when the bellows are racked out, for the operator to manipulate the fine adjustment. A silk cord, rubbed on a piece of resin to make it " bite," is sometimes passed round the milled head of the fine adjustment and carried by screw eyes to the back of baseboard. But the focussing rod attached to Messrs. Watson's student's camera, as illustrated, is a much more efficient means for this delicate adjustment.
Exposure.—At length we are ready to expose. Place the card in front of the condenser, to cut off the light and act as a shutter. Insert the dark slide, and expose the plate by withdrawing the card. The length of exposure must depend upon the illuminant, the amount of magnification, and the colour of the object. An orthochromatic plate is essential, and it must be backed ; fifteen seconds may be taken as the normal exposure, without the yellow screen, using the one-inch power.
Orthochromatism.—One of the most serious problems in photo-micrography, especially at high powers, is that of colour values, with a view to securing contrast and, at the same time, sufficient detail. Readers who have studied the subject of orthochromatics will readily understand that neither are likely to be satisfactory in the result, unless the object is photographed by light of the wave lengths com prised within its absorption band. The two rules to be observed are : (I) To increase the photographic intensity, or render it as black as possible, use a screen of complementary colour ; (2) to decrease the photographic intensity, that is to say, in order to render detail within the subject itself, use a screen of the same tint as the light it transmits. The application of these two rules will de pend upon circum stances, the nature of the background, and the variation of colour in the sub stance itself. As a preliminary method of determining by what light the maxi mum contrast is obtainable, Messrs. Wratten and Wain wright recommend visual examination under the micro scope, first by means of screens trans mitting light ab sorbed as completely as possible, and then by other screens in which the light is less completely ab sorbed. Such filter screens are usually fitted either to the stop holder of the condenser, or introduced into a special holder between the microscope mirror and the light, according to the system in use.
Vertical Cameras.—There are advantages in the vertical pattern of camera, one of which, from Messrs. R. and J.
Beck's numerous varieties of apparatus for the purpose of micrography, is illustrated here. It is very easily attached to the microscope, without previous preparation, and for objects of a fluid nature as, for instance, when photographing the inhabitants of a drop of ditchwater, the vertical form is the only one that will meet the case. Microscopes used in a vertical position have a tendency to gradually drop down wards during a prolonged exposure ; this difficulty the manufacturers have overcome by an adjustable stop-block to the coarse adjustment of the " London " microscope. For simpler souls there is a little fixed-focus camera, which has only to be pushed on to the eye-piece end of the microscope draw tube, after careful focussing of the object, and the whole arrangement is ready for exposure. It is quite efficient, of . light weight, and takes plates t: in.—not too small for making lantern slides.
our list of illuminants we omitted to mention one which for most photographic purposes is regarded as the cheapest and most efficient of all. And no doubt the best results in this branch may be obtained from sunlight if the worker will provide himself with a heliostat—an inexpensive pattern will often meet the case—in order to keep the sun's image concentrated on the object during exposure. For even in the short space of time required our earth goes on travelling steadily on its axis, and without this piece of clock work, the beam of light which we focussed with such nicety would creep away and expend its radiance elsewhere.
The work with the microscope is not impossible with a good instrument, and a large dia phragm shutter that will fit on to the barrel. The nimble proteus, and suchlike forms of life, have been successfully taken at sec. with a very rapid plate. For accurate results, the liquid should be placed in a Holman life slide some hours before the amcebm undergo the ordeal of portraiture, in order to accustom them to their surroundings. Needless to add, the exposure must be by limelight, or a very reliable arc burner.
Degree of Magnification.—A micrograph is of very little value unless the magnification has been ascertained. This is not a difficult task. Remove the slide just photographed from stage of microscope, and, without altering the focus, substitute a stage micrometer. With the aid of a divider, the comparative size of the image on the focussing screen and of the original is an elementary matter of calculation.
The best English works on the subject are Photo-Micro graphy, by E. J. Spitta ; Practical Photo-Micrography, by J. Edwin Barnard ; and The ABC of Photo-Micrography, by W. H. Walmsley.