2 Professor Airy on the Spherical Aberration 



the subject at all : and those who have made some steps, have 

 confined themselves to the simplest cases. I am not aware that 

 any writer has investigated the conditions which must be satisfied 

 that an object may be seen distinctly in all parts of the field of view, 

 though this is the most imiiortant point in the construction of 

 eye-pieces. 



The effects of spherical aberration may be described as follows. 

 The first is a distortion of the object. If, after examining an object 

 in the center of the field of view, we bring it to the outside, we 

 shall frequently find that it is extended in the direction of a radius 

 of the field of view, and that it is increased, though in a smaller 

 degree, in the other dimension. If we look at a square, it will 

 appear to be drawn out at the corners, so that the sides are all 

 convex towards the center. Sometimes the contrary effects will 

 be produced : an object being less magnified at the circumference 

 of the field than near the center. This defect may, in many cases, 

 be entirely removed. 



The second effect is, that if an object be distinctly visible in the 

 center of the field of view, it is necessary -to push in the eye-piece 

 farther, in order to see clearly the objects at the outride of the 

 field. In consequence of this, it is impossible, with the same 

 position of the eye-piece, to see distinctly all parts of the field, or 

 to see an object distinctly as it passes the field of view. This defect 

 can never be destroyed. 



The third eflfect is, that no adjustment of the place of the 

 eye-piece will make an object distinctly visible when it is far from 

 the center of the field of view. If a brilliant point, as a star, be 

 viewed in this situation, with one position of the eye-piece, it 

 appears a bright line in the direction of a radius of the field, and 

 with another position it appears a bright line in a direction per- 

 pendicular to the former : with other positions it appears an ellipse, 

 or a circle. In the case last mentioned, different parts of the last 



