DELICACY OF THE ADJUSTMENT FOR THIN COVERS. 
43 
through no more than half a division of its graduated scale. 
With this form of adjustment, the transition from good to bad 
definition takes place with such a slight movement of the col- 
lar, that with a little practice, the best point of definition is 
readily found. 
It often happens that the amount of balsam, or other me- 
dium, covering different objects on the same slide, varies, there- 
fore, the adjustment should be examined for each portion of the 
slide, where we wish to make the most accurate observa- 
tions. 
For object glasses of the largest angular aperture, very great 
care and patience are often required in effecting the most per- 
fect adjustment. With glasses of moderate aperture, if the 
adjustment has been well made for glass cover of medium 
thickness, the same correction will answer very well for ordi- 
nary observations, even should the thickness of the glass cover 
be slightly varied. For common investigations, therefore, 
much time may be gained by assorting the thin glass into par- 
cels of nearly uniform thickness, and having obtained a medi- 
um adjustment for one variety, so long as the same parcel of 
thin glass is used, the labor of adjusting for thickness of glass 
cover may be dispensed with. But when glass of another 
thickness is employed, or when a more delicate object is to be 
examined, or an objective of very large aperture is employed, 
the adjustment for thickness of cover must be examined and 
made as accurate as possible. 
Increasing the distance between the objective and eye-piece, 
by extending the draw-tube, causes disturbance of the balance 
of aberrations, and requires renewed adjustment by moving 
the graduated collar, in the same manner as for different thick- 
nesses of glass cover. 
When an object-glass is examined that has no correction for 
thickness of glass cover, it will usually be found that it has but 
very moderate angular aperture. Such is the case with almost 
all object-glasses made on the continent of Europe. 
61. Our §econd Class Objectives have a somewhat smaller 
angular aperture, and are mounted in a simpler style. They 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
