230 Mr. Airy on Achromatic Eye-pieces of Telescopes, 
published in the Memoirs of the Academy, and afterwards col- 
lected in his Opuscules Mathematiques, published 1764. And 
Euler, in the Berlin Memoirs for 1757, gave a great many theorems 
upon combinations of lenses: which, in a paper in the Turin Me- 
moirs, Vol. III, published in 1766, are applied to the achromatic 
eye-piece. These papers, with considerable alterations, were 
afterwards embodied in his large work on Dioptrics, which was 
published in 1769 and the two following years. 
It does not appear that any more original investigations 
were made, till the Abbé Boscovich in the Memoirs of the 
Academy of Bologna gave some new theorems which were 
collected in his Opuscula, published in 1785: those relating to 
the achromatic eye-piece are principally contained in the second 
Volume. The subject is treated with great mathematical skill, 
and the work bears strong evidence of considerable practical 
acquaintance with optical instruments: the rules given by him 
are in general well adapted to the use of the working optician : 
and are, in fact, the foundation of all that have since that time 
been published. ; 
During forty years which have elapsed since the investiga- 
tions of Boscovich were given to the world, no addition I believe 
has been made to his theory. The only English treatise on 
this subject that I have seen, is one published by Professor 
Robison in his Mechanical Philosophy, and in the Encyclo- 
pedia Britannica, Art. Telescope. Though this writer has not 
closely copied from Boscovich, yet he seems to have been guided 
entirely by his theory: his calculations appear to have been made 
in the same way: however I have seen no treatise, which on the 
whole seems better adapted to give clear ideas on the construction 
of telescopes, and useful rules for the assistance of workmen. 
From this treatise, and from the work of Boscovich, have been 
extracted the theorems given by Dr. Brewster in the appendix 
