THE MICROSCOPE. 87 
may be united into one, forming the solid eye-piece. The 
latter is the better and is at the same time more expensive 
form. It is best to have two eye-pieces (those lettered A and 
C by most American manufacturers being the most useful). 
The greatest attention should be paid to the selection of the 
objectives and, unless the purchaser be an experienced per- 
son, some expert friend should be called in to pass judgment 
upon the lenses submitted. The great points to be secured 
are absence of color, flatness of field, and distinctness of 
image. All objectives above a one-fourth should be provided 
with an adjustment for cover glass. The “angle of aperture” 
should be reasonably large, but “high angled”’ lenses possess 
no value corresponding to their high price except to “Diato- 
maniacs.” 
Objectives are rated by English and American manufacturers 
according to their focal length, this being the distance be- 
tween the object and the “optical centre” of the lens when 
an image is formed. Thus a }? inch does not have a quarter 
of aninch between the objective and the object, but between 
the optical centre (which is nearer the distal portion of the 
lens than is the mathematical centre) and the object. 
For the beginner, the most useful objectives will be an 
inch and a quarter or fifth. As he proceeds in his studies and 
his familiarity with the instrument increases, higher, lower, 
and intermediate powers will be necessary. 
Among the higher powers (¢ inch and upward) it is cus- 
tomary to have the lenses of the kind known as “immersion.” 
In these the end of the objective is wetted with a drop of 
water which forms a thin film betweenthe cover of the slide 
