Wenham, on a Binocular and Single Microscope. 19 
should be of a width only just requisite to include the 
half aperture of any object-glass, one quarter of an inch 
is quite sufficient ; it 
should not exceed this ^' 
for two reasons, first, that 
the greater the thickness 
of glass that the ray has 
to pass through, the more 
difference there will be in 
the magnifying power of 
the two bodies, and second, 
that a thick prism takes 
the ray more away from 
the centre of the main 
tube, and increases the 
convergence of the two, 
bringing the eyes nearly 
approaching to the dis- 
agreeable condition of a 
squint. 
Both the transmitting and reflecting surfaces of the prism 
should be accessible for the purpose of wiping, for any par- 
ticles or mildew adhering to the latter will prevent total 
reflection at the point of contact. If the prism is well made 
and polished, and of the smallest size possible for admitting 
the pencil, the difference between the direct and reflected 
image is scarcely appreciable, and with this standard of com- 
parison a faulty prism will immediately be detected. By 
pressing back the spring catch or stop on one side of the 
prism-slide, it can instantly be withdrawn altogether, and as 
quickly replaced. 
% 
