42 , THEORY OF THE MICROSCOPE. 



or, if the sine is substituted for the tangent, as is more correct 

 (vide p. 24), 



a/3 = 2 x '5 x -611 = -611 rum. 



The position of the eye-point is to a certain extent fixed before- 

 hand in every Microscope ; it must always be so chosen that, if the 

 eye is applied at the usual distance from the eye-lens, all the 

 emergent rays will contribute to the formation of the final virtual 

 image, so that the whole field of view appears equally illuminated. 

 The focal length of the field-lens must in every case be sufficient 

 to adniit of the formation of the image-point, corresponding to the 

 second principal point of the objective, 10 to 20 mm. at least above 

 the eye-lens ; for, since the latter increases the convergence of the 

 rays, the distance of the image-point in question is always some- 

 what greater than that of the eye-point. 



A diaphragm is applied within the eye-piece in the plane of 

 the image formed by the objective with the aid of the field-lens, 

 and in conjunction with the eye-lens determines the size of the 

 field of view. The diameter of the field is equal to that of the 

 diaphragm multiplied by the magnifying power of the eye-lens, 

 which has a different value according to the position of the eye 

 and the distance of distinct vision. 



1}. Tlie Ramsden Eye-piece. 



Modern Microscopes are usually furnished with the Campani 

 eye-piece above described, as it is generally considered to be the 

 most favourable for aplanatism. In special circumstances, how- 

 ever, other constructions of eye-piece may be applied with ad- 

 vantage, among which the best known is that of Kamsden. 



The Ilamsden eye-piece consists of two plano-convex lenses 

 having their curved surfaces facing each other. The distance 

 between them is so short that they act together like a doublet, 

 whose magnifying power is equal to that of a single lens of greater 

 curvature. The object must, consequently, be so adjusted that 

 the objective-image is formed slightly in front of the lowest lens 

 between it and the objective. 



The path of the rays through such a doublet may be traced with 

 given distances and curvatures of the refracting surfaces as in the 



