12 
THEOEY OF THE MICKOSCOPE. 
most delicate performance of the entire combination. Such a 
combination of lenses is called an aplanatiG object-glass or 
objective. 
22. Cause of Superiority of EInglisli and American Ob- 
jectives. The best English and American opticians seek the 
highest perfection in each objective, and hence their glasses 
cannot be separated and arranged in new combinations. While 
opticians on the continent of Europe, almost without exception, 
still make their achromatic object-glasses on the old plan of 
separating the combinations, using the several glasses sepa- 
rately, or uniting them in new combinations. 
Although French and German achromatic object-glasses may 
thus be furnished at a low price, their performance cannot 
equal that of the best English and American glasses, which 
will always be preferred by those who understand the philos- 
ophy of the microscope, and are capable of judging of its 
qualities. 
23. Compound Achromatic Microscope. In using the 
achromatic object-glass for microscopial purposes, it is usuady 
combined with another instrument known as the eye-^iece^ the 
two, together, constituting the compound — achromatic — micro- 
scope. The eye-pieces used for the microscope are the same as 
are employed for the astronomical telescope, and consist of 
two kinds. 
24. The Positive Eye-piece consists of two piano convex 
lenses, with their convex sides turned towards each other, and 
set at such a distance that their compound focus is in front of 
the first lens. An eye-piece of this construction has less achro- 
matic and spherical aberration than any single lens of the same 
power, but as it causes some renewed coloring and distortion of 
the image formed by the object-glass, it is only used in the 
microscope for special purposes. 
25. The Negative Eyc-picce, invented by Huyghens, for 
the telescope, is so named because it requires the image to be 
formed behind the first lens, the second, or eye-lens, only being 
employed to view the image. Two piano convex lenses are 
placed with their plane surfaces towards the eye, at a distance 
J. & W. GRUNOW & GO’S ILLUSTRATED 
