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II. — On the Measurement of the Angle of Ajperture of Object- 
glasses. By F. H. Wenham. 
{Read hefore the Royal Mickoscopical Society, November 1, 1876.) 
All methods hitherto employed for measuring the degrees of the 
angle of aperture of microscope object-glasses, have included rays 
from oblique pencils which constitute the entire field. These having 
greater divergence from the axis and being superadded to the angle 
proper, confuse angle of aperture with angle of field. 
This remarkable source of error appears to have escaped notice, 
and is sometimes so considerable as to show an excess in the highest 
powers of nearly twice the amount of aperture attributed to them 
by opticians. 
The angle of aperture in reality means, that the cone of rays 
should proceed from one approximate point in the field of view : for 
example, it cannot be doubted, that if the margin of a very minute 
diatom were to be enclosed by a screen impervious to light it would 
not only be seen with the full aperture, but also with improved 
definition on account of the exclusion of extraneous rays from the 
field of view. 
It is the admission of a portion of the rays from lateral pencils 
that gives an erroneous excess of aperture, by the usual methods 
of measurement, for the marginal rays of the lateral pencils having 
a greater obliquity than the outer rays of the central one, of course 
admit light beyond the true degree, and therefore they should be 
stopped off during the measurement. 
In order to confine the cone of rays to an approximate point in 
the focus of the object-glass, I have proposed that the light should 
be admitted through a very narrow slit cut in an opaque film duly 
set in the focus. There are, however, several difficulties in the use 
of this slit. In the first place, the measurement depends on the 
effect of the disappearance of light. This is not altogether a 
definite indication, and may give rise to dispute, as it may be asked 
whether the index is to be stopped at the first movement in the 
slit, or continued till the last trace of light remains visible? 
Secondly, it may be objected that however thin the material 
through which the slit is cut, if the space is exceedingly narrow 
very oblique marginal rays from a large aperture may be cut off. 
If the first objection is met by obtaining through a suitable 
eye-piece arrangement a telescopic image of the lamp flame, in 
order to see this distinctly the slit must be opened out to a width 
that will cause a considerable error in excess of spurious aperture, 
arising from the admission of some lateral rays. 
I have stated that " the narrower the slit, the more accurate the 
result will be." This means strictly that for absolute accuracy, we 
