HOW TO PHOTOGRAPH MICROSCOPIC OBJECTS. 
61 
pictures will be the inevitable punishment of remissness in 
these tedious but essential preparations. 
We have seen that the object-glasses in ordinary use focus the 
actinic rays behind the screen on which the visual focus is seen; 
the picture, therefore, if taken at the visual focus, will be dis- 
torted in a proportion corresponding to the distance between 
this and the actinic focus beyond. In the higher powers this 
distance is so minute that it may practically be neglected ; but 
in the l^-inch, the inch, and the -J-, some alteration will have 
to be made — either the focussing screen must be moved back to 
the point of actinic focus before a plate is exposed, or, which 
amounts to the same thing, the object-glass must be moved away 
from the object, so as to bring the actinic focus into the place 
of the focussing screen. This is done by means of the fine 
adjustment, and it will be well to see before commencing that 
the screw works readily on the slightest movement of the milled 
head. Many nights’ work may be lost, and false results obtained, 
through neglect of this precaution. The index-needle should be 
placed at the extreme left of the dial, and moved uniformly in 
one direction. 
The best object to choose for the lower powers is one covered 
with fine hairs at varying depths, such as the proboscis of a fly, 
or the tracheal rings of the cricket’s tongue may be preferred. 
A photograph is taken at the visual focus, and it will be found 
that other hairs than those focussed appear sharp and defined 
in the negative, whilst those which were sharpest on the 
screen are hazy and indistinct. If we can recognise any two of 
these, and ascertain, on looking through the microscope, how 
great a movement of the fine adjustment is required to alter 
the focus from one of these hairs to the other — from a hair 
focussed on the screen to one actually in focus in the picture 
— we shall have ascertained the correction or ee turning out ’” 
for the lens in use at the time, and careful note should be 
made of it, if the result is confirmed by the subsequent test of 
focussing as before, altering the fine adjustment to the ascer- 
tained amount, and taking a picture then which shall correspond 
in all respects to the focus which had been obtained on the 
screen. 
This appears simple in description, but in practice it is not 
so. Many experiments will have to be made, picture after 
picture will have to be taken before the portion focussed on the 
screen finds its exact counterpart on the collodion film. As 
some slight guide to the beginner, I may mention that our 1 i-inch 
Smith & Beck required of a revolution of the fine adjust- 
ment-screw; a -f-inch by the same makers required of the 
divisions marked on the card dial, equivalent to ^ of a revo- 
lution ; a A^-inch Smith & Beck required one division on the 
