56 Professor Airy on the Spherical Aberration 



image distinct as far as spherical aberration was concerned, may 

 be considered one of the happiest in the history of optical in- 

 struments. 



Another improvement, of which the utility is more extensively 

 felt, is the substitution of the four-glass eye-piece for that with 

 three glasses only. With the old three-glass eye-piece we have 

 found that at a given distance from the center of the field, the 

 confusion in the image was three times as great as if a single 

 eye-glass were used. In consequence, the field of view was 

 narrowed in these eye-pieces to a most inconvenient degree. In 

 the four-glass eye-piece, the confusion is not by any means so 

 great as if a single eye-glass were used ; and is not one-fourth of 

 that in the three-glass eye-piece. The field of view may, there- 

 fore, be made at least twice as broad, preserving the same degree 

 of distinctness; a comfort which every practical observer can 

 easily appreciate. 



But the four-glass eye-piece possesses another advantage : it 

 is achromatic. It is true that the three-glass eye-piece {Cambridge 

 Transactions, Vol. II. p. 246.) might be made achromatic by in- 

 creasing the distance between the second and third lenses: but 

 this, I believe, was never done. And the mention of this brings 

 me to another question. How far is the positive eye-piece pre- 

 ferable for the application of a micrometer, to the four-glass 

 eye-piece % 



We have seen that the positive eye-piece is superior to any 

 other for the removal of spherical aberration. But it is not 

 achromatic, and cannot be made achromatic {Cambridge Trans- 

 actions, Vol, II. p. 244.) ; and considerable inconvenience is oc- 

 casioned by this defect. I have seen several micrometer eye-pieces, 

 in which there was a degree of chromatic confusion, that would 

 have been intolerable in a common perspective-glass. When the 

 object is not far from the center of the field, it is much greater 



