NOTES ON PHOTO-MICROGRAPHY. Lr 
so arranged that the illuminating beam falls exactly on the mirror. 
The illumination of the object can now be adjusted and the image 
accurately focussed on the glass of the camera, by means of the 
ordinary focussing screws of the microscope. An ordinary dark 
slide is then substituted for the focussing glass and the plate 
exposed, the exposure being regulated by means of a card inter- 
posed between the source of light and the orifice of the slide. 
I am well aware that the use of the eyepiece is contrary to the 
usually accepted opinions on the question, but I have found it to 
be more advantageous for several reasons, for instance, it allows 
of only a short distance between the object and the focussing 
glass, thus enabling the focussing screws to be easily under the 
direct control of the operator ; it also allows of a considerable 
magnification being obtained without increasing the distance 
between the object and the plate, and when a strong light, like 
the oxy-hydrogen lime-light, is used it does not make the exposure 
a very lengthy affair ; it also does not appear to disadvantageously 
affect the definition. 
The microscope which I usually employ with the above arrange- 
ment is one of Seibert’s, fitted with an Abbé condensor ; the 
powers used are either Nos. O, 1V., or VIb objective and Nos. 1 
or 3 eyepiece, varying of course with the nature of the object. 
The most usual combination is No. IV. objective and No. 3 
eyepiece, which gives, with the draw-tube out, a magnification of 
about 450 diameters. With the lime-light and an Ilford ordinary 
plate this combination requires an exposure of 20 to 30 seconds. 
When an oil-lamp with reflector is used, the exposure must, of 
course, be longer, thus the above-mentioned combination requires 
an exposure of two minutes to give a good negative. Without an 
eyepiece a much shorter exposure is naturally required, thus 
Seibert’s No. VIb objective, corresponding to an English one- 
eighth, required 60 seconds exposure with an oil-lamp, and 
Seibert’s No. IV., corresponding to an English one-fourth, 
required 15 seconds only with the same source of illumination. 
Plates II. and III. show some examples of photo-micrographs 
obtained with the above described arrangement. They are from 
untouched negatives. 
