Angle of Aperture of Objectives. By F. II. Wenhmi. 323 
riment. I selected a ^ which worked as an immersion, as this 
position prevents confusion concerning other points of adjustment. 
The full aperture, as measured by the ’sector through a slide with 
water between that and the front lens, was 120^. The focal dis- 
tance as immersion was *041. The diameter of transmission on the 
surfaces of front lens was * 07, ascertained by allowing a drop of 
milk to dry on the front and measuring the diameter of the light 
spot from parallel rays entering from the back, using a low-power 
object-glass and micrometer eye-piece for the measurement. 
The field of view included a diameter of * 03 on a stage micro- 
meter. With the exact focal distance from the front lens as a 
starting point, it remained to ascertain what were the apparent 
apertures taken through various stops of definite diameter set close 
to the front, that could only admit the base of a cone of rays from 
an angle proceeding from the axial focal point up to a known 
diameter of stop. The arrangement that I use is a form of ad- 
justable slit, consisting of two strips of very thin platinum foil ; one 
piece is cemented on to a slip of thin plate glass, which is made to 
slide under two staples by a micrometer screw acting against a 
counter spring. The fixed strip of foil is attached to one of the 
staples, so that when the screw is quite home the edges meet. The 
various widths at which the instrument was set were measured 
under the microscope with an eye-piece micrometer. Having got 
the desired width, the object-glass to be measured was attached, 
and the body of the microscope lowered till the slit came in contact 
with the front lens, a drop of water having been placed over the 
slit to prevent undue refraction, and obtain more light. 
The apparent angles included by these limiting edges or stops 
were measured by the usual sector method, of rotating the micro- 
scope on a turn-table graduated into degrees, and ascertaining the 
vanishing point of a distant light ; or, preferably, by means of an 
examining lens at the eye-piece, for observing the disappearance 
from the field of an actual image. 
The real or true angles were estimated from the distance of the 
focal point, up to the known measure of the edges of the stop. 
Avoiding fractions of degrees, the following table gives the com- 
parative results : — 
Working 
I )iameter of 
Front Li ns. i 
False 
Aperture. 
True 
Aperture, j 
Working 
rHameter of 
Front Lens. 
False 
Aperture. 
True 
Aperture. 
o 
o 
o 
o 
•07 
120 
89 
•02 
68 
31 
•06 
118 
80 
•01 
59 
16 
•05 
116 
70 
•005 
53 
8 
•or 
103 
58 
•002 
50 
•03 
88 
45 
1 
