188 Angle of Aperture of Object-glasses. By F.H. Wenham. 
aperture this way, have been puzzled by the different inversions, 
and have moved the sector the wrong way ; this allows the oblique 
rays which should be cut off from the margin of the field to enter, 
and the effect of the screen will in consequence be nil. I have 
therefore applied a very simple remedy that will prevent mistakes. 
Above the concave examining lens (placed over the lowest eye- 
piece), I adapt a cap with a pin-hole stop, like that used in a 
Gregorian telescope. This greatly improves the distant telescopic 
vision, and increases the range, so that the screen can be seen 
on one side of the field. The cap is now removed, and the object 
is brought by the rotation on either side till the approach of the 
median edge causes it to vanish. I find it best to take the aper- 
tures by daylight sitting with the back to the window, and using 
for an object a vertical strip of white paper an inch broad, set a 
foot or so away. 
Colonel Dr. Woodward maintains that immersion angles greater 
than 90° are now obtained, and tells us that this “was several 
years since demonstrated in the ‘ Monthly Microscopical Journal ’ 
both by Professor Keith and himself ” (?).* In the last Journal he 
gives another demonstration, with a figure, by which arrangement 
a parallel beam of light is so restricted that it cannot enter within 
that angle, but passes straight on without refraction to the top of 
the slide or cover of an object in balsam, where of course it must be 
totally reflected ; but if water is introduced between the cover and 
a high-angled object-glass, light will enter, and the field become 
luminous ; but this proves nothing more than that the oblique rays 
I have specified, as including the extreme of the field of view, find 
entrance, and in this consists Colonel Woodward’s mistake. There 
is no need of any such special contrivances, which tend to com- 
plicate and confuse the question. If an object-glass is duly ad- 
justed on an object in balsam, it is quite clear, and follows as 
an absolute optical condition that if the rays emerge beyond at any 
degree less than parallel with the slide, the balsam angle within it 
must be under the critical angle, or below 82°. Now, if my opaque 
screen, formed as a slide, is substituted, the adjustment duly per- 
formed, and the oblique pencils cut off according to directions, the 
angle from out of the slide will always denote the angle within it. 
I have not yet met with any object-glass that possesses an angle of 
150° — in fact, angles as high as 100 J are rare, and what advantage 
is to be expected beyond 150° ? In my last paper I gave a table of 
the percentage of value for apertures. The value of the ray-collecting 
* In tlie ‘ Quarterly Journal of Microscopical Science,’ No. xii., July 1855, 
page 303, I described how I first obtained the full aperture on objects in balsam 
by means of an additional front lens to the existing combination. 
I mention this because the question has been argued as if I was ignorant of 
the principle, and denied that in any case an immersed angle beyond 82° was 
possible. 
