106 Measurement of angles under the Compound Microscope. 
r. Wenham seems to confine his attention to the fact that a 
ag portion of the rays froma balsam-mounted object are lost — 
by internal reflection. This, of course, I never meant to deny, © 
and in fact it is one obvious reason why balsam-mounted test 
objects are, as I long ago stated, far more difficult to resolve than 
when mounted dry. The loss of a portion of the rays in this man- 
ner, however, has nothing whatever to do with the present ques- 
tion, which is simply whether of the rays that do emerge, (and 
which make every angle with each other from 0° to 180°) more 
will be collected by a lens of large or small aperture. Certainly 
Mr. Wenham cannot deny that ‘the larger aperture will receive 
the larger number of rays, and if so, then my statement is fully 
confirmed that “large angles of aperture are as useful for balsam 
mounted objects as for others.’ 
The distinction I have silnded to above, between the intensity 
of illumination of the balsam-mounted object and the effect of 
large angle of aperture, is alluded to by Dr. Robinson in his pa- 
per On a new method of measuring Angular Aperture,’ ” where 
he states in a note that the effect of mounting in balsam “is in 
fact equivalent to reducing the aperture of the objective below 
100°, as far as illumination is concerned, though a much larger 
one may be required to take in the pencil. ” As to the question 
whether large angles of aperture are always seals, en one 
will be apt to decide according to the merits of his own glasses, 
1 can only say that I have as yet seen oe to aah me fear 
that I may have lenses of too much apertu 
: 
~ 
: XV.—On eer Adaptations of the Baas Mie 4 
; by Oapen N. Roop : 
THE compound microscope and some of the piecés oe if appara- 
tus which generally accompany it, can, in the absence of ale 
especial optical instruments, be made very conveniently to per- 
form their part ;. we pr opose to mention briefly a few of the uses 
for which it may be employed. 
S a goniometer, aX the sesenetness of the sree of lar, 
of microscopic bao if the ale pe a Saseaster of from 
vernier, minutes can conveniently be read of a reflect- 
ing goniometer not being at hand dent that the angles of 
large crystals can be measured with this all that is ne- 
cessary is, to bring the microscope to an upright position, to re- 
