NOTES AND MEMORANDA. 
243 
tages, and should be both respected and encouraged. How we have 
been forced to modify our opinions, since an angle of aperture of, say, 
150° in microscojncal objectives was considered absolutely unattain- 
able! No doubt the very men who honestly and firmly believed those 
things impossible which are now quite familiar, were as glad as any 
one, when they became convinced, by facts, that they had been in 
error. Equally gratified, probably, will be Mr. Wenham, when he 
shall see for himself that an angle of more than 82° can be attained 
through balsam. Within a few days, I have had a good opportunity 
to see a T ] 0 tli objective of Mr. Tolies’ make give an angle of 92° 
through balsam with tank arrangement of Mr. Wenham. I feel dis- 
posed, however, to let Mr. Tolies speak of this in his own words, the 
more so as my time is extremely limited : — 
Dr. Josiah Curtis, Boston, July 8, 1872. 
Dear Sir, — At my request, you were present recently when I 
measured the angular aperture of an immersion yV-inch objective 
when immersed in balsam. You verified the results gained at that 
time. We used the tank method of Mr. Wenham.* 
The T L-inch. tested, I stated to you, had in air angular ap. of 170° 
(upwards). 
In water we found the angle to be 110°-f-. 
In balsam the angle was fairly 95°, using petroleum lamp flame, 
thin, for light, in a darkened room. As you will remember I remarked 
that with sunlight I got two degrees more. 
It will, I know, be of interest to you, and I am sure to some others, 
to hear of results of test of angle in some other cases. I will, there- 
fore, set down here the angle of aperture found to pertain to some of 
my immersion objectives 
in balsam, in 
water, and 
in air. 
Air. 
Air. 
Water. 
Balsam. 
Single front 
.. ^ in. . 
170° .. 
120° .. 
87° 
_1 
170° .. 
110° .. 
88° 
Compound front 
1 
175° .. 
117° .. 
95° 
1 
175° .. 
105° 
.. r (high) 
172° .. 
100° . . 
88° 
Single „ 
.. Tin. . 
175° .. 
127° .. 
110° 
The varying differences between the water and balsam angles can, in 
a general way, be accounted for from the formulas of construction 
differing considerably, each one from any other. 
Of all these objectives the most effective (especially when its low 
power is considered) is the 1-inch of 110° in balsam. This is true of 
its use for objects mounted in balsam, as Ehomboides, small. But 
notably so as to its work on dry A. pellucida. My London specimen 
of this, received through the U. S. A. Medical Museum, is resolved 
into lines that shine, I may say. The illumination I used was 
petroleum lamp flame, no condensation. With the same means all 
the objectives show A. pellucida with the same illumination, but with 
a difference. 
With sunlight and a blue cell no doubt the higher powers would 
have their proper advantage. 
* See ‘M. M. Journal,’ August, 1871. 
