TESTING THE SPHERICAL ABERRATION. 151 



ones up to a certain distance from the margin, from which point, 

 also, the aberrant rays cut the surface of projection of the image. 

 The observer, therefore, looking through the eye-piece at the 

 image formed by the objective (the plane of the diaphragm 

 representing the image-plane), perceives, outside the margins a 

 and y of the illuminated image-surface, a glimmer or fog due 

 to the rays affected with aberration. 



The testing of the aberration can therefore be simply effected 

 by viewing under the Microscope a small luminous surface, or a 

 real or virtual image which acts as such a surface, taking care 

 that no light reaches the eye from the surrounding part. The 

 fainter, therefore, the glimmer or fog surrounding the microscopic 

 image, the more perfect is the correction of the aberration. 



We know of no better object than that which was employed 

 in principle by Mohl, and later also by Harting. A glass slide 

 is coated with an opaque layer of Chinese ink, in which fine 

 points or lines to admit light are made with the point of a 

 needle ; or the layer of ink is allowed to dry, and a number of 

 fine, sharply bordered cracks will be formed, which are preferable 

 for testing to the artificial ones. Better still, but not so easily 

 made, are the small silvered glass discs (as employed by Abbe), 

 upon which bands of coarse and fine lines are ruled. Opaque 

 and transparent parts here lie inexactly the same plane, and the 

 different bands afford the necessary gradations for testing the 

 various objectives. 



In practice the condition must be fulfilled, which every test- 

 object ought in general to satisfy, viz., that the cones of rays 

 proceeding from the illuminating points, should occupy the whole 

 aperture of the objective (a fact which Mohl and Harting have 

 apparently disregarded). For this purpose the diaphragm must, 

 with the lower powers, be removed or sufficiently enlarged, and 

 the mirror must be brought nearer ; but with the higher powers 

 a condensing lens or an objective of suitable focus must be applied 

 to increase the convergence of the incident rays correspondingly. 

 If the objective to be tested has, say, an aperture of 60, the 

 illuminating cone incident upon the glass slide should have an 

 aperture at least as large. 



The other objects which have been recommended by micro- 

 scopists for instance, the minute images of window-frames 

 formed by an air-bubble or a mercury-globule (on a dark ground) 



