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 6 the field of view: we have 
a =a, and from 
Fa 
ie 
The parallelism of the bounding ray gives 
@ = cotan 1’ x 2 = cotan 1’. — 
«©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, u 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 : 
of. Ff @-9), , Vert), 
op eee df 
ee ee 
Ease. f HES 
The condition of distinct vision gives 
V=u-4u; 
-9(@- 2) =f" -F a. 
Developing which, and fase terms affected with —- 
whence 
- —-, and 
d 
upwards, o=f' ee ei ; 
combining which with the value of a, we obtain a value of d’. 
This quantity is also in general 
Pe 
= ges 
