58 Professor Airy on tJie Spherical Aberration, §c. 



I do not know of any form which could be conveniently used, 

 and only mention this as one of the objects to which, in the ulterior 

 improvements of telescopes, the attention of artists might be pro- 

 perly directed. 



whose coefficient is ^i^ . — ^„ . When one lens is concave, or/ is negative, we can give 

 n (c-g-)- 



to this term a negative value as large as we please, and, therefore, we can make 2(F) as small 

 as we please, and, consequently, the equation 



2(K) + i2(i)=0 



can always be satisfied. By substituting for a convex lens, a convex and a concave lens in 

 contact, whose combined power is the same as that of the convex lens, we may always satisfy 

 this equation, without altering the form or the achromatism, in those eye-pieces where other- 

 wise it is not possible, as in the Huyghenian and four-glass eye-pieces. 



G. B. AIRY. 



Trinity College, 

 July 28, 1827. 



