and the Achromatism of Microscopes. 233 
minating it, he has an equation between the focal lengths of the 
lenses and their distance from each other. Though the prin- 
ciple of this process is general and not inelegant, yet the 
method of introducing and afterwards eliminating the distance 
of the eye from the last eye-glass, is not so good as might have 
been expected from Euler. In pursuing the theory, he has 
loaded it with generalizations and details, to such a degree as to 
make it almost useless. The thickness of the lenses, in some 
parts, makes his formule extremely complicated, though that 
might be safely neglected; in other parts, he has considered the 
effect of lenses of different sorts of glass, though no one would 
resort to that construction, when it is possible toe avoid it. And 
I believe that I do not exaggerate when I say, that, after a 
general acquaintance with the principles of chromatic dispersion, 
it would be easier for any one to form a theory for himself, 
than to select the parts that are useful from the book of Euler. 
The work of Boscovich is much better calculated to explain 
the principles of the achromatic eye-piece, than any of those I 
have before mentioned. And in his investigation he has pur- 
sued the natural course of tracing the axis of pencils of differ- 
ently coloured rays through the eye-piece, and making them 
emerge parallel. But his method is greatly deficient in facility 
and power. He finds the quantity of mean refraction where a 
pencil is incident on the first eye-glass, from which he gets the 
dispersion of the violet and red rays: he then finds the distance 
at which they are incident on the next lens, and the refractions 
there; and; continuing this process, makes the last emerging rays 
parallel. In some places he has expressed the dispersion by an 
algebraic symbol; in others, by its numerical value; and in 
some he has calculated the progress of a ray by a laborious 
trigonometrical process. And this is the last theory pretending 
to any degree of generality that has yet appeared. 
