( 106 ) 
V . — On Angular Aperture of Immersion Objectives. 
By Robert B. Tolles, Boston, U.S.A. 
The following table gives measurements of angle of aperture 
for several objectives — taken indiscriminately, not selected for the 
purpose — in balsam, in water, and in air. Several of them were 
made years since : — 
Objectives. 
Angle in Air. 
Angle in 
Water. 
Angle in 
Balsam. 
tV' \ 
Single Front / 
170° 
100° 
81° (+) 
Sun. 
Ts l 
Single Front / 
170° 
120° 
87° 
iV \ 
Single Front J 
170° (+) 
110’ 
83° 
>> 
At \ 
Compound Front / 
147° 
97° 
82i° 
” 
Compound F ront } 
175° 
117° 
93’ (+) 
"io \ 
Single Front / 
175° (+) 
113° 
95° 
Lamp. 
« 1 
Compound Front / 
►— 1 
^1 
00 
o 
120° 
97° 
Sun. 
Compound Front } 
155° 
106° 
00 
00 
o 
» 
T 1 
Four Systems / 
175° 
127° 
110° 
This table of measurements of angle in balsam, &c., furnishes 
proof enough of the correctness of my position so positively denied 
by Mr. "Wenham, first in ‘Mon. Mic. Jour.,’ No. xxxii., p. 84; again, 
ibid., No. xxxvi., p. 292; and thirdly, No. xl., p. 272, of the 
same. The criticism bestowed in these articles induced the measure- 
ments. Still, if nothing of attempt to deny the significance of that 
first experiment as described in this Journal, No. xxxii., p. 36, had 
appeared, the above results of actual measurements would have 
been interesting as data of practical angle of “ immersion ” ob- 
jectives when water or any medium of ref. index not higher than 
1 ' 549 that of balsam is used. But somebody is mistaken, and I 
must necessarily be “ personal ” in pointing out how I am not, and 
others are, in respect to what I have set forth on angular aperture. 
In the first place all agree that in the case of a balsam-mounted or 
dry-mounted object-slide, if of common glass in use, the light 
incident at the first outside surface of that slide at the extremest 
obliquity will be so bent in entering the glass as to make the angle 
of the pencil after this first refraction less than 82°, the extreme 
rays of the pencil having approached the glass at an incidence of 
more than 89°, and the pencil angle more than 178°. Not to 
