116 PROCEEDINGS OF SOCIETIES. 
rience; but I think it would be an advantage if this reflecting disc 
were made of parallel glass, that is, glass the surfaces of which 
are accurately parallel to each other. Iam hardly prepared to 
say whether the glass should or should not be of the ordinary 
thickness, but [ am not aware that it makes much difference. It 
is a fact that, if we take any number of ordinary rounds for cover- 
ing objects of the usual diameter—} or 3 inch—we find some 
of them to be of sensibly different thicknesses in different parts ; 
and when high powers are used I cannot help thinking that if 
any portion be wedge-like it will to a certain extent interfere 
with the perfect definition which can be obtained from the best- 
constructed glasses of very high powers. If these glasses are 
parallel glasses, of course, im the passage of rays from the object- 
glass up to the body of the microscope, the rays will all be 
refracted equally. Ifthe two surfaces are accurately parallel to 
each other every ray will be refracted parallel to itself, that is, the 
direction of the rays will not be in the slightest degree altered ; 
but if, on the contrary, this glass be wedge-like, of course a dif- 
ferent amount of refraction will take place, and there will be a 
certain amount of chromatic dispersion of every ray transmitted 
through the wedge to the eye-piece, and that, 1 think, might be 
found sensibly to interfere with the definition. 
Mr. Becxk.— With reference to Mr. Brooke’s remarks, I would 
state that any variation of thickness in the glass of the illumi- 
nator would be far more evident with the lower than with the 
higher powers. It is well known that in the case of binocular 
prisms for the microscope a bad prism will give a greater amount 
of error with low powers than with high ones. In connection 
with this illumination, I found, when examining some of the 
Diatomacez, that the definition was much improved by cutting off 
half of the aperture in the side of the illuminator; and it then 
struck me that a piece of semicircular glass with its diameter . 
across the field of view would answer best of all, but I found the 
loss of definition to be very great. Definition is also lost by 
cutting off any considerable portion of the aperture of the object- 
glass; and if I rightly understand the American plan, an opaque 
substance comes over a portion of the aperture of the object-glass. 
Mr. Loss.—Very little indeed—scarcely perceptible. 
Mr. Becx.—Then I cannot understand how sufficient illumina- 
tion is obtained. 
Mr. Rorrr.—I have tried Mr. Beck’s and Messrs. Powell and 
Lealand’s new illuminators, and I quite concur with the observa- 
tions already made, that the plan will be a useful one, especially 
for the examination of the Diatomacez, the minute structure of 
which is in many cases difficult to make out, by ordinary methods 
of illumination. With respect to the difference between the plan 
proposed by our English opticians and the American method, 
I have been told, that half the field was cut off by the opaque 
mirror which is interposed between the object-glass and the eye- 
piece in the American plan, which I consider to be a great objec- 
