ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
411 
The ordinary eye-piece is now changed for a projection eye-piece, 
set to the distance at which the sensitive plate is to stand ; the camera 
is attached, and the long focusing-rod coupled on. The image of the 
sun will be found in the centre of the ground glass. If it is not, the 
centering of the condenser must be wrong, and will require alteration. 
The sun’s image should be sharp at the edge, unless the sky is hazy. 
Any light fleecy clouds near the sun will be visible on the screen, almost 
as if an ordinary landscape lens were being used, and the effect when 
they drift across the sun’s disc is very curious. The image of the object, 
as seen against that of the sun, will be somewhat out of focus t but a 
slight turn of the focusing-rod brings it right. 
With sunlight I find it unnecessary to use anything for focusing 
except the ground glass. The image is so bright that the details can 
be sufficiently seen. With other less brilliant sources of illumination it 
is necessary to use other means ; and that which I have found most 
convenient is a Microscope eye-piece. The ground glass is removed, 
and replaced by a wooden slide, in the centre of which is a hole fitting 
the eye-piece. It should be so set that the diaphragm of the eye-piece 
is in register with the sensitive plate. Even a very faint image, when 
viewed through this eye-piece, is sufficiently visible to admit of focusing. 
The next step is exposure. I wish I could give you some rule by 
which to regulate exposure, but I find it altogether impossible to do so. 
Its wide range has already been indicated. From one second with bright 
sun, to an hour and forty minutes with diffuse daylight, is a “ far cry.” 
Exposure depends not only on the source of light, but on variations of 
that source. A winter sun, shining through an east wind haze, is very 
different from a midday sun in summer, when the sky is clear. Exposure 
varies, too, with the degree of magnification. A magnification of 1000 
diameters requires 100 times the time that one of 100 diameters requires. 
It varies with the width of the illuminating cone. It varies with the 
opacity or transparency of the object. It varies with the colour of the 
medium in which the object is mounted. A diatom mounted in Prof, 
van Heurck’s high refractive medium, which is of a deep yellow colour, 
requires at least six times the exposure that would be proper if it were 
mounted in balsam, all other conditions remaining equal. All I can 
tell you, therefore, is that a little experience, and a few dozen spoiled 
plates, of which notes have been kept, will enable you to judge, almost 
instinctively, what exposure is required. I always make two exposures 
on each object, one longer than the other, and thus have a double chance. 
As regards plates, I recommend you to use slow ones, and to deve- 
lope with hydroquinone. The usual difficulty, with most microscopical 
objects, is to obtain sufficient contrast, and this is most readily obtained 
on slow plates.” 
Frazer — On Photography as an aid in Ad atomical, Histological, and Emhryological 
Work. 
Report 59th Meet. Brit. Assoc, for the Advancement of Science, 1890, p. 639. 
Pringle, A. — Practical Photomicrography by the latest methods. 
New York, 1890, 8vo, 192 pp., 7 pis. 
