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 }A4+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 2b the axis minor of the small speculum, 
Ib. y LEH), AVS =9), 
fag) Fs fg’ 
2-5 (24); pment os 48 bie 
& 
The condition of vision is 
V=u+u, 
whence 
oe ya prss) pe, 
and the expression of x gives 
F 
Ped (3 i‘ car) 
whence 
f' Pree cde o 
2 J at as 
of get) ht gga) 130° 
which for F = 36 = 94 gives 
2b = 0:2222 = 2°67 inches. 
f' = 01575 =1:89 ,, 
M = 240. 
@ = 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: :: 
b= a(5 +o -u)s 9. 4 (os ey _ ) 
F Zi ‘ 
