THE CENTERING OF THE SYSTEMS OF LENSES. 83 



is, of course, simply the sum of both the effects, which we have 

 just treated of separately. 



In accordance with what we have already stated, it is therefore 

 quite correct to say that defective centering exercises an injurious 

 influence upon the microscopic image. If, however, we disregard 

 the small inclination of the images to the axis, this influence is 

 confined, in the first place, merely to the margin of the field of 

 view, and reaches its centre only in the case of stronger inclina- 

 tions (which, of course, ought to be avoided). The familiar 

 phenomenon, due chiefly to the action of the eye-piece, by which 

 the distinctness of the microscopic image diminishes towards the 

 edge of the field of view, may, in consequence of these aberrations, 

 be still further intensified. 



Finally, we will examine one further point, to which we shall 

 refer in discussing the Testing of the Microscope. We have to 

 show what changes in position the image of any object-point 

 undergoes through imperfect centering, if the image-forming 

 objective is turned round a given axis (P Q, Figs. 37 and 38). For 

 this purpose we start from the results obtained above, according 

 to which a displacement of the lenses generally causes a displace- 

 ment of the objective-image also. In the example given, where the 

 optic axis was moved "25 and '5 mm. to the right and left, this 

 displacement amounted to 1 mm. to the left. If we suppose the 

 objective to be furnished on the left 

 side with a sign, it is evident that 

 whilst it turns round the axis P Q 

 the displacement will always take 

 place in the direction which is de- 

 termined by this sign. The centre 

 of the circular objective-image there- 

 fore assumes successively the posi- 

 tions denoted by 1, 2, 3, 4, with 

 regard to the centre o (Fig. 39) of 

 the field of view (bounded by the 

 eye-piece diaphragm), in which we 

 may imagine crossed wires, a led, to 

 be extended. It therefore describes a circle, of which the point o 

 is the centre, and the distance of the displacement is the radius. 

 Similarly, every other point in the image (since its position relative 

 to the centre must remain the same) describes a circle of equal 



G 2 



