150 



TESTING THE MICROSCOPE. 



most influential ; its determination, and the discovery of the 

 precise action of the lens-combinations, together form one of the 

 most important problems, especially for the optician. 



In the first place, the question arises in practice, how the 

 presence of spherical aberration is manifested in the microscopic 

 image ; and, secondly, we have to decide whether it indicates an 

 under-correction or an over-correction. The problem demands 

 still further consideration, for it frequently occurs that the ob- 

 jective is under-corrected for the central rays (say, up to an 

 inclination of 10 15), but over-corrected for the marginal. 

 Since, however, the optician judges by the total effect, he considers 

 a lens-system as far as possible aplanatic, if either the larger 

 central part or, as more often occurs, the larger peripheral portion 

 of the incident cone of rays exhibits no marked aberration. 



The general action of spherical aberration may be shown 

 graphically. If a b (Fig. 89) is a self-luminous object, whose 



FIG. 89. 



cone of rays completely fills the aperture of the objective A B, 

 and if we assume that the rays emerging from a and b unite 

 after refraction for the most part in the points a and V (in the 

 Fig. this is assumed for the rays of medium inclination), while 

 the others converge towards points lying somewhat nearer or 

 further, and which are more or less separated, as shown in the 

 Fig., for a central and a peripheral pair of rays : the real image 

 of the point a appears therefore in a, and similarly, that of b in 

 1) ; consequently the images of the other points of the object 

 appear in the intermediate points between a' and &'. An image- 

 plane drawn through the points a and b f would, as the construction 

 shows, be met beyond these points by other rays, whose focal 

 points, according to the hypothesis, fall in front of or behind this 

 plane. These rays do not originate merely from the extreme 

 marginal points of the object, a and b, but also from the adjacent 



