104 WenhaM; on a Binocular Microscope. 
the two sides, these may be so arranged that both images are 
ultimately combined at the eye-piece. There would be no 
difficulty in working the glass to a mean thickness of 3^^^ of 
an inch. In this form the angle between the sides would be 
so exceedingly small that the chromatic effect considered as 
a prism would be inappreciable in the direct eye-tube. 
Another idea was to dispense with the rectangular prism 
and employ a wedge-shaped piece of glass, with the back 
silvered as shown by fig. 1, making use of the front and 
back surfaces for the two reflections 
and separation of the images. The 
wedge should be achromatic, and com- 
posed of angles of flint and crown as 
shown. But this could only be em- 
ployed singly in such microscopes as 
have the object-glass set at right angles 
with the body. This plan might be of 
use for viewing sun-spots in a telescope 
where a diagonal eye-piece is seldom 
objectionable. In a microscope the in- 
crease of light obtained by removing the 
reflecting prism would not probably be very appreciable. 
If two reflecting and transmitting surfaces be placed in 
close contact, both images will be combined at the same point. 
On this princple the following arrangement was tried : 
fig. 2j are two similar right-angled prisms, with their 
F/G. I 
a a. 
^1 F/G. 2.^ \h 
\ 
diagonals in contact : these are to be sufficiently tilted out of 
the plane of the microscope to throw the first reflecting sur- 
face out of the range of total reflection, and allow direct rays 
to be transmitted. The rays, h b, reflected from both the 
diagonal planes, after leaving the side of the prism «, were 
thrown into the second body of the microscope by means of 
another right-angled prism, c, while the transmitted rays 
