NAMES APPLIED TO OBJECT-GLASSES. 
21 
bination and the object will be increased ; hence the ante- 
rior combination will act more powerfully, and the entire com- 
pound objective will have a certain excess of positive aberra- 
tion. 
Now as a piece of flat and parallel glass placed over an ob- 
ject produces chromatic and spherical aberration, both nega- 
tive, it is evident that it may be corrected by diminishing the 
distance between the anterior and middle combinations of the 
objective, till the positive aberration thereby produced in the 
object-glass balances the negative aberration caused by the 
medium. 
This correction is essential to the performance of object-glass- 
es of large angular aperture. Glasses of moderate angular 
aperture may have their corrections balanced for a medium 
thickness of cover, and then they will perform very well if the 
thickness of glass or other medium covering the object is va- 
ried within moderate limits. The mechanical arrangement of 
the object-glass, by which [the correction is made, to adapt it 
for viewing objects covered with different thicknesses of glass 
or fluid, is fully explained at section 58. 
34. Tlie magnifying power of the ]TIicro§cope is varied 
by the use of Eye-pieces of different poAvers, which are 
designated as Nos. 1, 2 and 3, No. 1 having the least magni- 
fying power. Nos. 1 and 2 are those generally furnished with 
our microscopes. No. 3 is of still higher power, and is only 
furnished when specially ordered. 
35. Magnifying power varied by Draw-tube. The 
power of the microscope may be greatly increased not only by 
using different eye-pieces and object-glasses, but also by increas- 
ing the distance between them by means of the draw-tube, with 
which the better instruments are supplied. 
36. Names applied to Object-Glasses. Object-glasses are 
usually distinguished by the focal length of a single lens that 
would give the same linear magnifying power ; but the dis- 
tance between the object and the anterior combination of a 
compound achromatic objective, is far less than its designation 
would imply, for its optical centre, from which its true focus 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
