76 



OPTICAL PRINCIPLES OF THE MICROSCOPE. 



FIG. 9. 



concave of flint; and at the back, another triplet, consisting of 

 two double-convex lenses of crown, with a double-concave of 

 flint interposed between them. By the use of this combination, 

 an angular aperture of no less than 170 has been obtained with 

 an objective of l-12th inch focus; and it is obvious that as an 

 increase of divergence of no more than 10 would bring the ex- 

 treme rays into a straight line with each other, they would not 

 enter the lens at all ; so that no further enlargement of the aper- 

 ture can be practically useful. 



15. The enlargement of the angle of aperture, and the greater 

 completeness of the corrections, first obtained by the adoption of 

 Mr. Lister's principles, soon rendered sensible an imperfection in 

 the performance of these lenses under certain circumstances 



which had previously passed 

 unnoticed ; and the important 

 discovery was made by Mr. 

 Ross, that a very obvious dif- 

 ference existed in the precision 

 of the image, according as the 

 object is viewed with, or with- 

 out a covering of talc or thin 

 glass; an object-glass which is 

 perfectly adapted to either of 

 these conditions, being sensi- 

 bly defective under the other. 

 The mode in which this differ 

 ence arises, is explained by 

 Mr. Ross as follows. 1 Let o, 

 Fig. 9, be any point of an object ; o P the axial ray of the pencil 

 that diverges from it ; and o T, o T', two diverging rays, the one 

 near to, the other remote from, the axial ray. Now if G G G a 

 represent the section of a piece of thin glass, intervening between 

 the object and the object-glass, the rays o T and o T' will be re- 

 fracted in their passage through it, in the directions T R, T' R' ; 

 and on emerging from it again, they will pass on towards E and 

 E'. Now if the course of these emergent rays be traced back- 

 wards, as by the dotted lines, the ray E R will seem to have issued 

 from x, arid the ray F/ R' from Y ; and the distance x Y is an 

 aberration quite sufficient to disturb the previous balance of the 

 aberrations of the lens composing the object-glass. The requi- 

 site correction may be effected, as Mr. Ross pointed out, by giv- 

 ing to the front pair (Fig. 8, i) of the three of which the objective 

 is composed, an excess of positive aberration (i. e. by under-cor- 

 recting it), and by giving to the other two pairs (2, 3) an excess of 

 negative aberration (i. e. by over-correcting them), and by making 

 the distance between the former and the latter susceptible of 

 alteration. For when the front pair is approximated most nearly 

 to the other two, and its distance from the object is increased, 



1 " Transactions of the Society of Arts," vol. li. 



