108 On Angular Aperture of Immersion Objectives. 
For persons not “ familiar with optical science,” and for others, 
Mr. Wenham’s tank method furnishes a ready means of test of 
angle in balsam, thus ascertaining the degree of fitness of any 
objective for viewing objects balsam-mounted. It proves, moreover, 
by simple experiment what many persons would not take the 
trouble to study out from a diagram and description, and the 
experiment is conclusive, at all events as to the point at issue 
between Mr. Wenham and myself. The measurements given at 
the head of this article for angle of objectives in water and in 
balsam were made strictly according to that tank method as con- 
tributed by him, only sunlight was used in most cases. The 
differences in the results are plainly referable to differences in the 
objectives as to plan of construction. There is no need to set forth 
the manner in which the result is produced in one case or another ; 
the fact remains of comparatively large angle for immersion ob- 
jectives over what is possible for “dry.” In proof that image- 
forming rays were concerned, I name the fact that every objective 
recorded in the table readily shows the true test, A. pellucida, by 
nncondensed light of a common petroleum lamp flame. In this 
connection let it be noted that most of the objectives in the list 
have the compound front of crown and flint glass, while some of 
them of equal angle have the single lens front. 
The i" of 110° in balsam has four systems substantially on the 
plan figured in ‘ Mon. Mic. Jour.,’ No. xxxix., p. 127, Fig. 1. 
It is easy to carry the angle on this plan much beyond 1 1 0°, but 
that breadth of angle, anything above 82° of course, is enough to 
dispose effectually of this extraordinary assumption and defy. 
“ I challenge anyone to get, through the object-glass 
with immersion front, a greater angle, or any portion of the extra- 
neous rays that would in the other case be totally reflected, as no 
object-glass can collect image-forming rays beyond that limit.” * 
One word about Fig. 1 (dotted lines) and Fig. 2, p. 117, No. xxxix., 
this Journal. Mr. Wenham says they are grossly erroneous. He 
is at fault. In the text, this in qualification and explanation, is 
stated as follows : — “ What is intended is to increase the refraction 
of the front by sharper convexity or higher refractive material or 
both to the extent necessary to make up for the diminution at the 
front surface which takes place according to the refractive power 
of the medium flowed in ” between the front and covering glass. 
Higher refractive material involves refraction at the front plane 
surface, of course and necessity. 
As to the practicability of the first case illustrated by the same 
figure, involving four systems in the objective, that seems to be 
admitted. It is not denied. I understand Mr. Wenham to admit 
it, in effect, in two of his three replies. The i" in the table, of 
* Mr. Wenham, in ‘M. M, J.,’ No. xxvii., p. 118. 
