XIV. On the Principles and Construction of the Achro- 
matic Eye-Pieces of Telescopes, and on the 
Achromatism of Microscopes. 
By GEORGE BIDDELL AIRY, B.A. 
FELLOW OF TRINITY COLLEGE, OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY, 
AND CORRESPONDING MEMBER OF THE NORTHERN INSTITUTION. 
[Read May 17, 1824.] 
Iw the theory of Telescopes no part is more interesting, and 
in the practical construction scarcely any more important, than 
the Achromatic Eye-piece. The effects of a badly formed eye- 
piece are even more disagreeable to the eye than those of a defective 
object-glass: whether we consider the indistinctness near the 
extremity of the field of view, the distortion of the image, or the 
fringes of colours which surround an object when not observed 
in the center of the field. Important and interesting as the subject 
appears, we might expect, in works of the highest pretensions, 
to find it treated in the comprehensive manner it deserves: and 
simple as are its principles, we might imagine that they would 
be introduced into our elementary treatises on Optics. But it is 
remarkable that while all writers have given at great length the 
theory of the achromatic object-glass, there are not more than 
one or two books in the English language, in which the achromatic 
eye-piece is alluded to; and though not a telescope has been made 
except on this construction for many years, the artist is still obliged 
Vol. Il. Part II. Ge 
