Royal Microscopical Society. 
255 
that the |~inch aperture, or the holes £ inch apart, represent the 
1-inch disk, the |~inch aperture a 2-inch disk, and the l|-inch aper- 
ture a 5-inch disk at ten inches distance. In the latter cases the 
result would have to be divided by 2 or 5 to obtain the power. 
The next point is as to the mode of measuring the reduced 
image in the eye-piece. For low powers the ordinary divided scale 
dynamometer may suffice ; for higher powers a double-image dyna- 
mometer, or one with a compound microscope, would be necessary. 
With high powers a measurement with the lowest eye-piece would 
suffice, as the relative powers of the eye-pieces can be very exactly 
obtained : the mean of several observations should be taken. It may 
be as well to subjoin an example to show the method of working : — 
Disk lj-incli at 24 inches = 5-inch disk at 10 inches distance. 
Diameter of image ‘028) 5 '00 (178 '6 power nearly. 
I venture to think that this method will be found well adapted 
for a standard of measurement. It is simple, showing the power 
with the smallest amount of calculation, whatever may be the foci 
and construction of the objective and eye-piece, and length of body, 
adjustment for cover, &c. If the body is made, as before suggested 
(by means of suitable adapters), to measure ten inches from the 
front surface of each objective to the stop in each eye-piece, the 
standard will agree with the real measurement ; and even if the diffi- 
culty of measuring the minute disks of light be considered too great 
with high-power objectives, even approximations may be useful, if 
based upon a standard the conditions of which can be easily re- 
membered and reproduced. 
There are two uses of the optical portion of the dynamometer, 
which, though long known, seem to have been too much overlooked 
of late years. In the form of Pioss’s centreing eye-piece, viz. a 
positive eye-piece of moderate power, means are afforded of ascer- 
taining very exactly the condition of the inside of the tube, the 
positions of stops, existence of stray light, extent and centricity 
or obliquity and direction of illumination, &c. The other use is 
that of obtaining the real worhing angle of aperture of an objective 
when direct light is used and the eye -piece is in position, and not 
merely the angle at which a burr of oblique light- may be just 
visible. This plan was used in 1862 in examining the objectives at 
the International Exhibition.* 
If a double-image dynamometer is used, it must be ascertained if 
astigmatism exists in the eye of the observer. This defect (which 
is much more common than is generally supposed) will greatly 
vitiate the measurements, unless corrected by the use of proper 
(cylindrical) lenses. 
* See 4 Quarterly Journal of Microscopical Science,’ IV., N.S., 1804, p. 84. 
