194 



THEORY OF MICROSCOPIC OBSERVATION. 



in coming to the conclusion that under these conditions the whole 

 are incident upon the effective portion of the objective. 



Hence the point P, since the entire cone of the incident rays 

 contributes to its image, appears equally bright with any other 

 point of the field of view. The like holds good, of course, with 

 respect to all the points that lie nearer to the centre, except that 

 for these the refraction is less, so that the emergent cones of light 

 meet a more central part of the anterior surface of the objective. 



The centre itself is illuminated by rays 

 j which traverse it without refraction. 



On the other hand, at the points 

 further from the centre, as, for instance, 

 p (Fig. 105), a diminution of the light 

 is perceptible. That is to say, for them 

 the angle of emergence ( = that of in- 

 cidence) a, as a glance at the figure 

 demonstrates, is greater, and conse- 

 quently p ( = 90 [a a] ) is less. 

 Hence a marginal ray parallel to T L 

 (Figs. 104 and 105) will be so refracted 

 that it deviates more than 30 from the 

 perpendicular, and is therefore lost to 

 view. This diminution of course takes 

 place as well for the point immediately 

 adjacent, and as we get nearer and nearer 

 to the periphery, it becomes also ex- 

 tended to the rays of the cone of light situated more towards the 

 left, until at last only the marginal ray t /, parallel to TL' t reaches 

 the eye. All rays not lying in the plane of the paper are 

 ineffective for this limiting position. 



The determination of these limiting points is made in exactly 

 the same manner as that of the point P. If p is half the angle 

 of deviation, the equation is 



co + 8 



T 



FIG. 105. 



90- 



therefore in the given case 



p = 90 - 



60 + 30 C 



IFrom the relation above obtained a a = 90 p we get 



22J. 



a a 



