SIMPLE MICROSCOPES. 
the object can be placed at a much greater distance from the 
anterior lens n n, and can consequently be more conveniently 
Fig. 12. 
manipulated, if it be desired to dissect it, or to submit it to any 
other process; it can also be illuminated by a light thrown upon 
that side of it which is presented to the glass ; this could not be 
done if it were nearly in contact with the glass, which must 
necessarily be the case by reason of its very short focal length, if 
a single lens were used. 
30. It was recommended by Dr. Wollaston, the inventor of 
these doublets, to give the two lenses com¬ 
posing them unequal focal lengths, that of Pfe* l 3 - 
E e being three times that of D r. * ^^ 
The lenses are usually set in two thimbles, 
one of which screws into the other, as shown in m m 
fig. 13, so that they can be adjusted as to their ».,! 1§L^ 
mutual distance, so as to produce the best effect. 
When still higher powers are sought, the lens D d is replaced by 
two plano-convex lenses, in contact, which taken together play 
the part of the single lens D D in the doublet; this combination 
is called the triplet. 
When a very low magnifying power is required, the lenses E E 
and n d may be separated, by unscrewing. 
#1. The lenses, whether of a doublet or a triplet, being thus 
properly mounted, expedients must be adopted to enable the 
observer to apply them conveniently to the object under examina- 
109 
