Probably the most convenient and generally useful form of camera is one with bellows extending some thirty inches, and capable of carrying both quarter and half sized plates, horizontally or vertically. My box is a square one, carrying plates from the 4-4 size (6½x 8½ inches) down to 3¼x4¼. The bellows are in three divisions, extending fully four feet, giving altogether nearly six feet from the microscopic stage to the focussing screen when fully drawn out. It is very carefully made of hard wood, and the framework carrying the bellows moves with the utmost smoothness upon its A-shaped runners. A short cone front receives the microscope tube, and all extraneous light is shut out by a wrapping of black velveteen around the opening, secured by a rubber band. The focussing screen (which is only used for arranging the object in the centre of the field or plate, and is entirely removable) has a space of the precise size and shape of a lantern slide pencilled in its centre, as a guide to the making of quarter plate negatives. An object filling this space on the ground glass will necessarily occupy the same position on the quarter plate when the holder containing the latter is made to take the place of the focussing screen.
As stated, the ground glass is used only for containing the object, and for carse adjustment of focus. It is impossible to grind it finely enough to accurately focus any delicate tissue with high or even moderate powers. Many suggestions have been made and devices used, but all I have tried proved unsatisfactory, until the following appeared in an English journal:
An evenly coated gelatine plate is to be exposed to a flood of white light, developed to a very slight intensity - a mere smokiness - fixed and washed as usual, then bleached with mercury, washed and dried. The result is the most perfect focussing surface imaginable. To use, remove the focussing screen from the camera and replace it with the plate holder from which the slides have been removed. The bleached gelatine plate (which of course must be of proper size to fit the holder) is to be placed therein with the film side toward the microscope. Now place a focussing glass against the back of the plate, apply the eye to it and adjust the focus by a delicate pull of the cord. Nothing can be more satisfactory. When the proper adjustment is determined upon, secure it from possible change during exposure, by winding the cord once or twice over the screw eye at the rear of the camera frame.
The plate holder (single) opens at the back to receive the plate, and being square, like the camera box, admits of the negative being taken vertically or horizontally, as may be desired, a matter of no small importance in many cases.
This feature, in connection with the revolving stage, enables one to place any object upon the plate in the best position for printing. The full size of the plate carried by my holders is 6½x8½ inches, but by the employment of removeable rabbett kits, plates 5x7, 4x5 and 3¼x4¼ can be used. It is desirable to have two or three additional holders as time savers.
For our work to possess any real uniformity or value, it is absolutely necessary that the magnifying power used with each exposure should be ascertained and noted. To make measurements each time would be a waste of that precious commodity, to avoid which I have adopted the following device:
The bed or framework upon which the bellows extends, is divided into spaces of one inch, and the same plainly marked thereon. Suppose this extension is thirty inches, and that by means of the stage micrometer we find the one and one-half inch objective to magnify twenty diameters with the bellows closed and fifty fully extended. This gives the value of one diameter to each inch of bellows drawn out, and if the magnified object reaches the desired dimensions, at, say twenty inches extension, we at once know the power to be forty diameters. Suppose, again, a power of one-fifth is being used, magnifying 125 diameters with bellows closed, and 275 with the same extended thirty inches; this gives a value of five diameters to each inch, and if the desired size is reached at twenty-five inches extension, we have a power employed of 250 diameters. By making these measurements (closed and open) for each objective habitually used, and recording the same in our note-book, an accurate and readily adjustable table of measurements is ready for all future work.
We now pass (thirdly) to the important subject of illumination, in many respects the most important in photo-micrographic work.
It goes without saying that to the favored few, whose time and means permit the harnessing of the sun's rays in their service, no artificial light can be produced which will quite take the place of the great luminary, but for the many, some other means must be sought.
When the promised days arrive that are to place in every man's house electric lights, cheaper than gas and as easily controlled, the problem will be satisfactorily solved; but at present we must content ourselves with prosaic kerosene, seeking only for the most practical method of utilizing its brilliant light.
After many experiments I have settled upon a flat, broad-based lamp of good oil-holding capacity, which is attached to a heavy retort stand with movable arm, enabling it to be carried to any desired height. A duplex burner and a tall chimney, producing perfect combustion, afford a light of thirty candle power, rendering possible the following average exposures with the plates I am now using, the variation in time being caused by differing densities and actinism of the tissues or substances to be photographed: