256 MEMORANDA. 
they will correspond to some figure on line pc, being now 
tens of the 1000 scale, as they were units of the 100 scale. 
Thus have we a ready means of fulfilling several important 
requirements ; first, of measuring the elevation for the camera ; 
secondly, of accommodating or adjusting one power with 
another; thirdly, of proving the reputed powers of different 







lenses ; fourthly, of proving the precision of micrometers ; 
fifthly, of drawing the images of objects in exact numerical 
terms; and lastly, of detecting errors of observation. 
The accompanying scale is only half the size of that re- 
quired. Gunter’s scale represents a fullinch.— Wn. Henpry, 
Surgeon, Hull. 
Angular Aperture.—I beg to submit an account of a simple 
method by which any one may easily measure the angular 
aperture of his object-glasses. I can hardly suppose it is 
original; but as I have met with no account of it, and as it 
dispenses with the use of the protractor, and of the appa- 
ratus described in the books for the purpose (without which 
some, I know, are not aware that the object in view can be 
effected), I venture to send it, in the hope that it may be of 
use. 
Place the microscope horizontally, with the eye-piece 
removed, and the object-glass attached whose aperture is to 
be measured. On a line (which may be conveniently in- 
dicated by a piece of string stretched across the table on 
which the microscope stands) at any moderate distance from 
the object-glass, and at right angles to the axis of the in- 
strument produced, place two candles, having their fiames, as 
nearly as may be (which may be easily managed by the aid 
of a few books), on a level with the axis of the instrument. 
Now, with the eye applied to the ocular end of the micro- 
scope, make an assistant slowly move first one candle and 
then the other along the line on which they are placed, till 
the images of the two flames are just seen at opposite edges 
of the object-glass. Measure carefully, with a bit of string, 
first the distance between the two flames, and secondly the 
distance from the object-glass of the line joining them. The 
