THEORY OF THE MICROSCOPE. 



surfaces, since it diminishes the ratio of the lens-thickness to the 

 radii of curvature or to the optical effect. With equal power of the 

 objective, the focal distance is, therefore, the larger, ccvteris paribm, 

 the higher the refractive index of the substance of the lens ; with 

 equal curvatures the magnifying power increases with the refractive 

 index, while the object-distance varies according to circumstances. 

 To assist in the estimation of these influences we have collected 

 in the following table the most important elements of our objective- 

 system for the cases given in the first two columns. The form of 

 the lenses and the refractive index of the flint (1/6) are here 

 regarded as constant, but the distances e l and c 2 of the double-lenses 

 (e l between the anterior and middle, c 2 between the middle and the 

 last) as well as the refractive indices n v n y and 7i 3 of the crowns 

 are regarded as variable. We will hereafter select from these 

 combinations that which appears best adapted for the correction 

 of the aberrations. 



2. THE EYE-PIECE. 



Eye-pieces are, in the first place, intended so to refract the diverg- 

 ing pencils of rays, which form the real objective-image, that they 

 may all arrive at the pupil of the observer's eye. They have also 

 to form a virtual image (as perfect as is possible under the given 

 conditions) of the real image, which is presented to them as the 

 object. It is, therefore, plain that a single lens will fulfil the latter 

 condition in a satisfactory manner only for the centre of the field 

 of view, and on account of chromatic aberration will produce a con- 

 fusion at the peripheral points, since the pencils of rays emerging 

 from these points pass through the margin of the lens. For this 



