periscopic Camera Obscura and Microscope. 375 
same scale to any desired length. Since the first line BC was 
laid down parallel to AF, and equal to mr, any other lines drawn 
through corresponding numbers 7 and 7, 8 and 8, &c. will be 
also parallel, and by preserving due proportion, will correctly 
represent mr. Hence in all positions of the line FR, the same 
similarity of triangles obtains, and the same proportion of F : 
R : : mr : R — r ; and consequently the focal length, corre- 
sponding to any assumed radii, is truly ascertained. 
For the purpose of duly proportioning the curvatures of 
flint-glass, a second line Ay might be laid down in a mode 
similar to the preceding, by adapting the multiple 
= — to the different density of this glass. 
With respect to the construction of a microscope on peris- 
copic principles, I believe the contrivance to be equally new 
with the former, and equally advantageous. The great desi- 
deratum in employing high magnifiers is sufficiency of light ; 
and it is accordingly expedient to make the aperture of the 
little lens, as large as is consistent with distinct vision. But if 
the object to be viewed, is of such magnitude as to appear 
under an angle of several degrees on each side of the centre, 
the requisite distinctness cannot be given to the whole surface 
by a common lens, in consequence of the confusion occasioned 
by oblique incidence of the lateral rays, excepting by means 
of a very small aperture, and proportionable diminution of 
light. 
In order to remedy this inconvenience, I conceived that the 
perforated metal, which limits the aperture of the lens, might 
be placed with advantage in its centre ; and accordingly I 
procured two plano-convex lenses ground to the same radius, 
3 C s 
