27 
«« The Newtonian does not require the same exactness 
in its arrangement, but I add the formula for it. The data 
are, that the image must be outside the tube, distant from 
the axes 34+ m+o-u; m may be neglected if the tube 
be iron; the eye must receive the whole pencil; and the eye- 
stop, placed at the image of the large speculum, must be 
V distant from the last image of the object. With a single 
lens, calling 26 the axis minor of the small speculum, 
Fy LEH), , AVES'-8) 
Sat aE nif ap? ah 
rf 
2 = 5 (24); nano 
gee ee 
uU= 
The condition of vision is 
V=ur+4, 
whence 
, , F-+ 2f' 
Ref (rss) pO, 
and the expression of x gives 
; F 
ics ¢ ~ 6A r) 
PE A a al 
whence 
154 * 900d?) ~ 120° 
which for F = 36 = 9.4 gives 
2b = 0:2222 = 2°67 inches. 
f' =0-1575 = 1:89 ,, 
M = 240. 
6 = 8°37. 
For the Huyghenian the formule given for the Cassegrain 
apply, changing d’ into F, and a, when it belongs to the small 
speculum, to A: 
A tan | 2b - 
m= 5(F + #-)5 0 (oH 
Se. RS 
