154 
Fig. 1. 
cj 
CROWN 
-""""^ CROWN 
FLINT 
a 
CL 
C 
G 
On an Improved Binocular Microscope. 
By F. H. Wenham. 
(Read June 13th, 1860.) 
In a paper read by me before the Microscopical Society^ in 
May, 1853, and published in Vol. II of the ^ Transactions/ 
I described several arrangements for obtaining binocular 
vision with the microscope, and stated that I had obtained 
the best definition by means of an achromatic prism, shown 
by fig. 1. a a is a prism of flint-glass ; bb two separate prisms 
of crown-glass, cemented there- 
on; cc is a ray of light, inci- 
dent on the flat surface of the 
flint prism. At its final emer- 
gence from the crown, it is re- 
fracted outwards f without colour 
or distortion, in the direction 
shown. If this compound prism 
is placed behind an object-glass 
with the line of junction coinci- 
dent with the optic axis, it will 
separate the pencil of rays emanating from the object, and 
give two images — that obtained from the right and left-hand 
half being brought respectively into each eye on the same side. 
In Dr. Carpenter^s ' Manual of the Microscope,^ the 
faults of this instrument are thus stated precisely : This, 
too, was far from being satisfactory in its performance, having 
two capital defects ; namely, first, that the view that it gave 
was often pseudoscopic J the projecting portions of the object 
appearing to be depressed, and vice versa. And, second, that 
the two bodies being united at a fixed angle of convergence, 
the distance between their axes could not be conveniently 
adapted to the varying distances of the eyes of different indi- 
viduals.^' I have since entirely removed these causes of ob- 
jection, by slight modifications not detracting from the ori- 
ginal simplicity of the instrument. When two stereoscopic 
pictures are accidentally so mounted as to give a pseudoscopic 
effect, the remedy is to transpose them. For a similar reason, 
I have transposed the images in my former binocular micro- 
scope, by refracting them so as to cross each other immedi- 
ately behind the object-glass, bringing the right-hand system 
of rays into the left eye, and vice versa. Fig. 2 illustrates 
the mode of accomplishing this : a a are two prisms of flint- 
glass, cemented to a single four-sided prism, b, of crown- 
glass. A ray of light, cCj incident on the surface of the flint 
