212 Transactions of the Royal Microscopical Society. 
II. — On the Measurement of the Angle of Aperture of Microscope 
Object-glasses. By F. H. Wenham, F.B.M.S. 
( Read before the Royal Microscopical Society, October 3, 1877.) 
In a communication to this Society, read November 1S76, I stated 
that angle of aperture is strictly the angle of a cone or pencil of 
rays starting from the axial centre of the focal plane, up to the 
diameter of transmission of the object-glass, and that all lateral rays 
from pencils of greater obliquity, extending to the margin of the 
field of view, have been hitherto included in measurements by the 
usual methods, thus indicating erroneous results far in excess of the 
true angle. In that paper I described the construction and use of 
an opaque screen, with a straight-edge set in focus, which edge was 
to be brought up to the axis or centre of the field alternately, 
during the measurement, so as to cut off all lateral raj s, and confine 
the fight to an absolute fine or point in the focus. 
As great difficulty was afterwards experienced in adjusting and 
using this screen, and mistakes were liable to occur from taking 
the readings in the wrong direction, I afterwards proposed, in the 
‘ Monthly Microscopical Journal,’ to use a pin-hole stop, set over 
the “ examining lens ” (which is used above the lowest eye-piece for 
obtaining a distant telescopic image), in order to confine the eye to 
the axial directing the instrument ; but the plan was subsequently 
shown to be objectionable, as in conjunction with the half-screen set 
at the same time in the focus of the object-glass, the aperture of 
this was found to vary with the size of the stop at the eye end ; 
and the plan was altogether so troublesome, that the preference has 
been given to a method of greater simplicity. 
In order to obtain a correct indication of angle of aperture the ex- 
terior rays, including the angle of field, must be cut off. This can be 
effected with greater facility at the eye-piece, for every oblique ray, 
including the field of view at the object-glass, is reproduced as field 
of view at the eye-piece ; therefore a small stop or aperture placed 
here will also exclude lateral pencils, at the final emergence, corre- 
sponding with those at the focus, without diminishing the aperture 
or axial angle of the object-glass : the result is more certain and 
accurate on account of the increased magnitude of the pencils. 
The arrangement that I now make use of, consists of an “ ex- 
amining lens” placed over the lowest eye-piece. This lens is a 
plano-convex achromatic of near four-tenths of an inch focus, con- 
tained in a tube sliding in an outer one, fitting firmly on to the eye- 
piece nozzle ; at a distance of one and a half inch behind the lens 
there is a removable cap, containing a thin plate with a central stop 
of one-fiftieth of an inch in diameter. The small size of this stop 
and the distance that it is placed from the lens, ensures the fixed 
