298 Dr. Herschel's observations of the satellites 
I have occasionally availed myself of the light of concave 
eye glasses, but a great objection against their constant use 
is, that none of the customary micrometers can be applied 
to them, since they do not permit the rays to form a focal 
image. Their very small field of view is also a considerable 
imperfection ; in observations, however, that do not require 
a very extensive field, such as double stars or the satellites of 
Saturn and the Georgian planet, this inconvenience is not so 
material.* 
As I have already shown that the effective power of a tele- 
scope arises from the combination of its magnifying and space 
penetrating powers ; and have also proved that the effect of 
their union, when they are differently combined, must have 
a considerable range, it will now be easy to point out the 
extent of this range in the telescope by which the following 
observations have been made. 
The magnifying power by which the satellites of the planet 
were discovered was only 157 ; but this power, which has been 
crossing, jostled against each other, or were turned aside from their right lined course 
by inflections or deflections. With a view to this, I directed a io feet telescope to 
some finely engraved letters put up at a convenient distance. A convex eye glass was 
fixed to a skeleton apparatus, which left the focal point freely exposed. A concave 
mirror was placed so as to throw the focus of the sun’s rays upon the focal image of 
the telescope, where, meeting with no intercepting body, they would freely pass 
through it at right angles. Then a screen being placed to keep off the solar rays, 
I fixed my attention upon the letters viewed in the telescope, and the screen being 
alternately withdrawn and replaced, I could perceive no sensible alteration in the 
brightness or distinctness of the letters. Hence I .surmised, that the rays of light did 
not sensibly jostle in an instantaneous right angled passage, but that possibly they 
might suffer inflections or deflections in their crossing at the focal point on account 
of their being longer in collateral proximity. 
* See Phil. Trans, for 1794, p. 58. 
