MICROSCOPICAL APPARATUS. 45 
on account of their short working distance. It 
must be remembered that a lens which has barely 
enough working distance to enable the operator 
to focus through the thin glass cover of the prepa- 
ration he desires to photograph, when the eye- 
piece is used, will fail entirely when this is 
removed, and an attempt is made to project the 
image upon a screen removed to a distance of 
several feet. For the farther the sereen is removed 
Srom the object, the nearer the objective must approach 
wz. Objectives of moderate angle have the advan- 
tage of greater penetration and ample working 
distance. These are therefore to be preferred 
for ordinary use, and it is only when resolving power 
of a high order is required for special work, that 
the more expensive wide-angled lenses will be 
essential. 
Flatness of field and good defining power are 
qualities which every first-class objective should 
possess; and when the Author a few lines back 
advised beginners to use such microscopical ap- 
paratus as they might possess, he referred to 
objectives of this kind, and not to the cheap toys 
which abound in the market, for it would be a 
waste of time to attempt to do anything with 
them. First-class objectives made ten or fifteen 
years ago, very suitable for photography, may 
often be obtained second hand at a very mod- 
erate price, as the tendency of late years has 
been to seek wide-angled lenses. 
The following objectives by well-known makers 
are recommended, — the four-inch, two-inch, one- 
