56 
Wenham, on a Method of 
cemented on to the under surface of the slide with Canada 
balsam ; d, is an Amici prism for condensing and directing 
the rays into the prism c; e, is a large bull's-eye condenser 
placed with its convex side towards the lamp. 
Fig. 1. 
Making ample allowance for all possible differences of 
refraction in the slide, balsam, and cover, the angle of total 
reflection for the mean refrangible ray, will vary from 40° to 
45° from the perpendicular — at any rate it will never exceed 
the latter degree ; consequently for this reason I consider the 
right-angle prism the most convenient for most purposes, as the 
rays may be passed perpendicularly through its surfaces with- 
out any trouble arising from refraction. 
The mode of action illustrated by the diagram is simply as 
follows : the rays from the luminous source are first collected 
and converged by the large bull's-eye lens e, and then further 
condensed and directed upwards by the Amici prism d; they 
next enter the surface of the right angle prism e, and pass 
directly onwards till they reach the upper side of the thin 
cover, from whence they are totally reflected down again, 
forming a brilliant surface of light, which will of course illu- 
minate any small bodies immersed in the balsam just below. 
If the cover is clean and free from scratches, not the smallest 
portion of light from the luminous source will find its way 
through. The view of the objective will be unimpeded, and 
the field perfectly black. Another way of causing the light to 
enter the prism, is by means of a parabolic condenser, adjusted 
as under ordinary circumstances ; the light will in this case 
enter the two faces of the prism at the same time, which is 
some advantage ; it must be sufficiently small to have some 
