Angle of Ajperture of Object-glasses. By F. H. Wenham. 287 
the edge only that intercepts the extraneous rays. The top might 
touch the object-glass without detriment to the result. However, 
it is more convenient to employ a film as thin as possible for the 
slit. This may be easily made as follows : — Take an ordinary 
3x1 glass slip, pour upon it a drop of turpentine (which is best 
after being long kept), and drain it off ; move the slide about over 
the open smoking flame of an ordinary coal oil lamp, till the black 
deposit is quite impervious to light. Again pour turpentine over 
the cooled slide, drain, and evaporate it dry by heat. The film 
may now be cut to a very clear edge with a sharp penknife, drawn 
along the edge of a small square. Make two cuts about one- 
twentieth of an inch asunder, and scrape away the intermediate 
black with a pointed wire, guided by the straightedge. Lay over 
the slit a thin glass cover, and let Canada balsam run under it by 
capillary attraction. The small particles left in the slit are excel- 
lent objects to adjust the object-glass by, the aperture of which is 
taken in the conditions of its actual use, and for an immersion lens 
water can be used between that and the cover. 
It is not difficult to bisect the field by estimation, but if required 
a cross line may be inserted in the micrometer slit of the eye-piece. 
The plan of obtaining an angle of aperture by measuring the 
working portion of the front lens by the diameter of the spot of 
light transmission, and distance of focus from the surface as the 
data for the angle, is only so far useful for demonstrating that 
certain stated angles are impossible, for the computed result may 
be excessively inaccurate. For example, an object-glass may have 
an extensive field with good definition with no focal distance, the 
plane of this being on the surface of the front lens. In this case, 
of course, the resulting angle will come out near to 180°, however 
small it may in reality be. 
Finally, in order to show the fallacious value of pretended 
angles near to this extreme, I append a table of apertures: the 
relative value of the degrees is taken as the chord of the arc. The 
limit of 180° is indexed as 100, and the corresponding figures give 
the comparative percentage of value for each aperture. 
Relative value of angular aperture taken 
as the Chord of the Arc. 
180° = 100- 
170° = 99-6 
160° = 98-4 
150° = 96-5 
140° = 93-9 
Relative value of angular aperture taken 
as the Chord of the Arc. 
130° = 90-6 
120° = 86-6 
110° = 81-9 
100° = 76-6 
50° = 42-2 
By this table ft will be seen that there is but little to be gained 
by apertures exceeding 150°, while the last lines of the column 
show the great increase of value for a corresponding increase in 
degrees. 
