ACHROMATIC MICROSCOPES. 
CHAPTER I. 
THEORY OF THE MICROSCOPE. 
1. Introduction. In calling public attention to the claims 
of Compound Achromatic Microscopes, and their great superi- 
ority over all others, it is necessary to explain the defects of 
simple lenses, the general structure and action of achromatic 
lenses, and also the entire optical and mechanical arrangement 
of the most improved Compound Achromatic Microscopes. It is 
presumed that every reader of this catalogue understands, or 
can easily learn from ordinary works, the simple elements of 
optics, and the structure of simple and compound miscroscopes 
formed of simple lenses. 
2. Simple liCiises. The names applied to the various forms 
of simple lenses sufficiently explain their structure, and are 
generally understood. All lenses are supposed to be bounded 
by plane, or spherical, surfaces ; for although some other forms 
would be desirable if they could be made with sufficient accu- 
racy, it is found in practice that only plane and spherical sur- 
faces can be wrought so perfectly as to render them available 
for optical purposes. 
3. Foci of Fenses. A piano convex lens E E, Fig. 1, 
(or any spherical lens thicker in the centre than at its edges,) 
refracts parallel rays of light L L, to a point F, called its 
1 
