126 Animal Life 
cardboard is removed for, say, two seconds to allow the light, artificial or otherwise, to 
shine through, thereby ensuring a pure white background, so essential in this work. 
Before having the apparatus constructed you must decide what will be the greatest 
amount of enlargement you 
are likely to vequire, as it 
is sumply a matter of length 
of bellows. We will suppose 
your lens has an equivalent 
focus of five inches. In this 
case, to photograph a specimen 
natural size the bellows must 
have sufficient “stretch” to 
‘ allow a distance of ten inches 
between the plate and the 
optical centre of the lens, the 
specimen being the same dis- 
tance from the lens. But to 
enlarge up to, eg., eight 
diameters, the plate must be 
45 inches from the centre 
‘ of lens and the specimen 
52 inches; therefore you will 
note that for enlargements of 
eo many diameters a long “stretch” 
is essential. No guide can be 
given as to requisite exposure, 
so much depending on the 
amount of enlargement, the 
colour of the specimen, speed 
of plate, class of illuminant, 
etc. An isochromatic plate I 
find the most satisfactory, 
and sometimes a yellow 
P X . - 
ie \ screen is necessary, especially 
. if the specimen is _ highly 
a coloured. ‘The frames holding 
the glass plates (1 and 2) are 
about five inches apart—thus, 
in the case of an unmounted 
imsect, avoiding the bugbear 
shadow. 
In some instances it is not 
advisable, or indeed possible, to unmount a specimen from its card. In such cases 
special care must be given to the lighting, or bad shadows will result. Experience 
is the only guide. 
STALK-EYED FLIES. x &. 
