10 
Wenham on Binocular Vision. 
binocular vision applied to the microscope, is that next to be 
described, in which I have altogether dispensed with reflecting 
surfaces, merely using three refracting prisms, which, when 
placed together, are per- 
fectly achromatic, a a, fig. 9, 
is a single prism of dense 
flint glass, with the three 
surfaces well polished. h b 
are two prisms of crown 
glass, of half the length of 
the under flint prism, to the 
upper inclines of which they 
are cemented with Canada balsam. The angle of inclina- 
tion to be given to the prisms must depend upon the dispersive 
power of the flint and crown glass employed. In the combina- 
tion that I have worked out, I have used, for the sake of sim- 
plicity, some flint and crown that Mr. Smith kindly furnished 
me with, in which the dispersive powers are exactly as two to 
one, consequently I have had to make the angle of the crown 
just double that of the flint, in order to obtain perfect achro- 
matism. The refractive power of each must also be known, 
that we may determine the angles of the prisms suitable for 
refracting the rays from the object-glass into the two eyes, at 
a distance of nine inches, c c, fig. 9, represents a ray of light 
incident at right angles upon the under surface of the flint 
prism. On leaving the second surface and entering the 
crown prism it is slightly bent inwards, and on finally emerg- 
ing it is refracted outwards, in the direction required. 
On looking through this prism I could not discover the 
slightest colour or distortion ; it is almost like looking through 
a piece of plain glass, and the loss of light is so inappreciable, 
that it is difficult to distinguish any difference between an 
object and its refracted image. 
The base of the compound prism should not be larger than 
is sufficient to cover the stop of the lowest object-glass, in 
order that they may be made very thin. 
The method of applying the prism to the binocular micro- 
scope is shown by fig. 10. a a is the object-glass, b the prism, 
placed as close behind it as the fittings will admit. The 
prism is set in an aperture in a flat disc of brass, which has 
an horizontal play in every direction, in order that it may be 
adjusted and fixed in such a position that the junction of the 
prisms may bisect the rays from the object-glass, and at the 
same time be at right angles to the transverse centres of the 
eye-pieces. 
c c are the two bodies of the microscope, provided with 
