70 



THEORY OF THE MICROSCOPE. 



Let a b (Fig. 33) be the field-lens image, in which the diminished 

 cones of rays (which are represented in 

 the figure by simple lines) converge to 

 the point o; let A B be the eye-lens, 

 whose centre of curvature is in o; and 

 let the eye of the observer be adjusted 

 for infinite distance. Then, since the 

 pencils converging to o are incident in 

 the direction of the radius, they undergo 

 no deviation at the lower surface, and the 

 refraction is confined to removing the 

 point of convergence to an infinite distance 

 in the direction of the axis of the pencils. 

 The virtual image a V resulting from this 

 (which is to be considered at an infinite 

 distance) 1 agrees, of course, completely 

 with the object, i.e., there is a perspective 

 coincidence of the single object-points 

 with their images. Eectangular net-work 

 should therefore appear rectangular also 

 in the image. Nevertheless, this virtual 

 image is considerably curved, because the 

 marginal points of the object are further 

 from the refracting surface than the 

 FIG. 33. centre. 



By the second refraction at the upper surface of the eye-lens 

 the parallelism is obviously not destroyed the image remains at an 

 infinite distance. But the pencils are bent aside from their course 

 the more in proportion to the angle which they form with the axis 

 of the Microscope. The amplification which the virtual image 

 thus undergoes is expressed approximately by the refractive index. 

 It is not, however, uniform throughout. If we suppose the 

 object a b to be divided into any number of equal parts, the 

 pencils proceeding from the points of division meet the eye-lens 

 and (if produced backwards) the plane of projection of the virtual 



1 Our figure is accurate for the assumed finite distances. The virtual image 

 is in this case brought nearer in consequence of the second refraction by the 



( 1 J -th part of its former distance, in which n denotes the refractive index. 

 For the others, the course of the rays is exactly the same. 



