Wenham^ on a Binocular Microscope. 
105 
passed through straight into the direct tube. The perform- 
ance of this arrangement was very satisfactory, an equal 
amount of light being apparently obtained in each eye. 
The next plan is to be preferred, shown in the diagram 
four times the full size, a a, fig. 3, in outline, resembles the 
now well-known form of Wenham prism," but in order to 
transmit light the first reflecting angle, 6, must be within the 
1 > 1 \ 
< \ 1 
a 
1 a : 
1 ' I 
range of total reflection. This for crown glass is when the 
ray makes with the surface an angle of 48° 11', and allowing 
due margin for difl'erence of density, the incident angle should 
not be less than 50° with the plane, consequently the angle, 
h, should be inclined 40°. The back angle, c, must be 
arranged in accordance with the inclination given to the 
bodies of the microscope, but as a rule the difference be- 
tween the angles of the front and back surfaces is half the 
angle of deviation of the ray from the perpendicular. 
In the microscope for which this prism was made, the 
bodies are set relatively at 15°; the back of the prism is, 
therefore, shown in the diagram 47-^°. This also falling 
within the range of total reflection must have its surface 
silvered. On the first inclined surface of the main prism is 
adjusted the prism d, with two polished surfaces making an 
angle of 40°. With each other this brings the top of the 
supplementary prism parallel with the base. The rays from 
the object-glass are transmitted without any displacement 
into the direct tube. A portion of the same rays are reflected 
from both contiguous surfaces, and thence from the silvered 
back into the slanting body. The two prisms are fitted into 
a small drawer of the usual size which slides into the opening 
of the ordinary binocular microscope. The two prisms need 
not be pressed into contact — if so, Newton rings are formed ; 
they may be set a visible distance asunder, but great care is 
