HOW TO WORK WITH THE TELESCOPE. 
281 
Subjecere oculis distantia sidera nostris, 
AEfcheraque ingenio supposuere s uo. 
In the first place, it is necessary that the beginner should 
rightly know what is the nature of the instrument he is to 
use, and this the rather, because, while it is perfectly easy to ' 
obtain such knowledge without any profound acquaintance 
with the science of optics, yet in many popular works on 
this subject the really important points are omitted, and even 
in scientific works such points are too often left to be gathered 
from a formula. When the observer has learnt what it is that 
his instrument is actually to do for him, he will know how to 
estimate its performance, and how to vary the application of 
its powers — whether illuminating or magnifying — according 
to the nature of the object to be observed. 
The astronomical refracting telescope, then, in its simplest 
form, consists of a large convex lens of long focus called the 
object-glass, and a small convex lens of short focus called the 
eye-glass. These two glasses are placad as at A B and a b in 
fig. 1, the distance between them being the sum of their focal 
lengths. Now, let the arrow E M F be supposed to represent 
a distant object ; * then an inverted image fme of this object 
is formed at m, the focus of the object-glass, and by means of 
the eye-glass a b this image can be microscopically examined. 
* The object is supposed to be very much larger than EMF, and so far off 
that the bounding rays from A and B (shown in the figure) meet at the point 
corresponding to the point P of the object. 
