6 
Wenham on Binocular Vision. 
a larger size, as he states that "The outer prisms can be 
cemented to the inner by Canada balsam." This amounts 
to the same thing as using a pair of prisms of solid glass, 
such as is represented by cc, fig. 1. I have carefully tried 
both of these methods, and find that the prisms alter the 
chromatic correction of the object-glass, and also materially 
injure the definition; for in making arrangements of prisms 
of this description we must always bear in mind that they 
produce a similar kind of aberration as a piece of glass of the 
same thickness as the distance which the ray passes through, 
Y\cr, 5. both before and after its reflection. There 
is also great difficulty in getting a per- 
fectly flat surface to the small reflecting 
planes. All these defects will be greatly 
magnified by the eye-piece, 
I have also tried what effect could be 
produced by means of plane reflectors, as 
Mr. Riddell says, " 1 use, for lightness 
and economy, four pieces of common 
looking-glass instead of prisms." My 
experiment was not tried with common 
looking-glass, but with thin microscopic 
covering-glass, silvered at the back. The 
definition with the lower powers was to- 
lerably good, but the loss of light very 
great. 
In order to remove the illusion of ele- 
vations appearing as depressions, Mr. 
Riddell proposes the "additional use of 
erecting eye-pieces ; " but I am afraid that 
when the microscope is taxed with this 
addition, the loss of light and defining 
power will become very great, and that 
even easy test-objects will appear so ob- 
scure as to preclude all hope of our 
making any additional discovery relative 
to their structure. 
I must remark, that I have made these 
last observations and experiments merely 
for the sake of arriving at the truth, and 
not with the view of detracting in the 
slightest degree from the merits of Mr. 
Riddeirs invention; for very great credit 
is no doubt due to him for leading the 
way to the practical application of a principle, in the absence 
of which the microscope still remains an imperfect instru- 
ment ; and, for my own part, I may, in all probability, shortly 
