96 
PRACTICAL DIRECTIONS. 
has been prepared with especial reference to the greatest effi- 
ciency for this class of observers. It is also furnished at a 
very moderate price. 
Our advice is often solicited by persons who have a limited 
amount of money to expend for a microscope, and who desire 
to know what selection would be most useful for their several 
purposes. For those who design to devote but a limited amount 
of time to the use of the microscope, we would recommend 
one of the smaller microscopes, 'No. 1, 2, or 3, with the appara- 
tus usually attached to them as stated in our Price List. 
To those who desire to have at once an instrument suited 
to the more delicate investigations at the smallest cost, we 
would recommend the Student’s Microscope, Ho. 3, furnish- 
ed with First Class Ohjectives.^ to be purchased according to 
their means in the following order: For the study of botany 
and mineralogy, the 1 inch, J inch, ^ inch, 2 inch, J inch, 
inch objectives. For studying entomology, anatomy and 
pathology, the -J- inch, i inch, 1 inch, J inch, 2 inch, ^2 i^^^h 
object-glasses. 
The Student’s Larger Microscope, Ho. 4, furnished with 1 
inch, -J- inch, and i inch objectives, Bull’s-eye Condenser, 
Camera Lucida, and Micrometer, forms a very complete instru- 
ment with which almost all vegetable and animal tissues and 
fluids can be examined satisfactorily. This combination is 
especially recommended to Medical Students, and all others 
who intend to devote considerable attention to microscopic in- 
vestigations, and desire to make the most economical invest- 
ment of limited means, and yet secure s, first class instrument. 
124. equalities of Objcct-Crlasses. In considering the 
value of an object-glass, and its adaptation to any particular 
purpose, several distinct qualities are to be examined, viz: (1.) 
its defining power ^ or the power of giving sharpness of outline, 
especially on the borders of an object, or where dots or lines 
are examined ; (2.) its resolving power., by means of which 
closely-approximated markings are distinguished ; (3.) fiatness 
of field ; (4.) depth of definition., which refers to the distance 
above and below the focus, that parts of an object can be seen 
with tolerable accuracy. 
1. Defining power ^ in addition to perfect workmanship, depends principally 
upon the perfection of the corrections, both for Spherical and for Chromatic aber- 
ration, The difficulty of securing this perfect correction increases with the 
angular aperture. Any inaccuracy in adjusting the object-glasses for thickness of 
glass cover, which every observer must arrange for himself, is more conspicuous 
in glasses of large angular aperture. For this reason, object-glasses of moderate 
J. & W. GRUNOW & CO’S ILLUSTRATED 
