MAGNIFYING GLASSES. 
sighted eye ; e' (fig. 3) a short-sighted eye, and e", (fig. 4, a far¬ 
sighted eye. Let the same small object, L it, be placed at the same 
Fig. 2. 
Ik 
m 
distance from each of them, and let the distance of most distinct 
vision for the first be e/; for the second e' l and for the third 
Fig. 3. 
e" V. If, by the interposition of a lens, the object L m be rendered 
distinctly visible to each of these three eyes, it will appear at l m 
Fig. 4. 
r 
to E, at V m! to e', and at V' m" to e" ; its apparent magnitude, 
therefore, to the three eyes will be in the exact ratio of their re¬ 
spective distances of most distinct vision, and consequently the 
magnifying power to e' will be less, and to e" greater than to E. 
It must, however, be observed, that in this, which is the com¬ 
monly received explanation, a circumstance of some import¬ 
ance is omitted, which will modify the conclusion deduced from 
it. To produce distinct vision with a given lens, L l, the dis¬ 
tance of the object from the lens will not be the same for diffe¬ 
rent eyes; for short sight the object must be nearer, and for long 
sight more distant than for average sight. 
Now, if this variation of the distance from the lens, or of the 
focus, as it is called, for different eyes vary in the same proportion 
as the distance of most distinct vision (and it certainly does not 
differ much from that proportion), it will follow, contrary to the 
received doctrine, that the magnifying power of the same lens, will 
be the same for all eyes, whether they have average sight, long 
sight, or short sight. 
102 
