24 
the first lens, A, a, a’, their apertures; d and d’ the distances 
of the first and second images from the second speculum; M 
the magnifying power, and @ the field of view: we have 
a =a, and from 
Le 
The parallelism of the bounding ray gives 
@ = cotan 1’ x a cotan 1’. x a 
“The simplest ocular arrangement is to view the last 
image with the unaided eye. As, however, this telescope re- 
quires an eye-stop, or aperture, placed so as to transmit no 
light except what comes from the small mirror, a lens must 
be used to form an image of that at the stop. This lens will 
also form an image of the second image. Let ¢ be the dis- 
tance of the second image behind the lens, w that of the third 
image in the same direction, wu’ that of the image of the small 
mirror, and 2 the section of the central pencil at the eye-stop : 
eR Cine) PN TA CI 
“= p) aT) 3 RS a ae © 
o+f d-o-f df 
Eee art es 
Be aE 
The condition of distinct vision gives 
V=au -u; 
whence : 
GP OF) et ee 
Developing which, and omitting terms affected with = and 
upwards nda 
combining which with the value of z, we obtain a value of d’. 
This quantity is also in general 
A Fa 
ii a a 
