164 PROCEEDINGS OF THE SOCIETY. 



" in suitable cases perform better than dry ones. With an appro- 

 " priate fluid interposed the critical angle for glass becomes ob- 

 " literated ; and the pencil of rays, whatever its aperture, can pass 

 " direct to the posterior surface of the front lens, and there 

 " become refracted for effective use in forming an image ; and 

 " this is where the immersion lens has the advantage ; it can 

 " include as large a portion of the radiant pencils of light as 

 " the lens is constructed to admit, irrespective of the mounting of 

 " the object, but in no case can it reach, far less exceed, 180°." 



Fig. 34. 



/70° IN AIR 



"Fig. 34 represents the first position, as defined by Mr. Shad- 

 bolt — a pencil of 170° radiating from an uncovered object in air. 



Fig. 35. 



I70 JN AIR 



\ ~~ ~ ~"""""^--^^' 



"Fig. 35 represents the second position, — the object in haham, 

 with a cover-glass, the pencil, at the object, being only 80° * (though 

 expanding to 170° m air). 



" Now if it is not recognized that the smaller angle of 80° in 

 balsam of Fig. 35 is the exact optical equivalent of the larger angle 

 of 170° in air of Fig. 34, it is easy to fall into Mr. Shadbolt's error 

 and to look at the matter thus : In order to make the angle at the 

 object in Fig. 35 equivalent as an angle to that of Fig. 34 (so that, as 

 it is supposed, the same things may be discussed in both cases !), a pencil 

 of 170° in balsam is taken as radiating from the object, as shown by 

 the dotted lines. All of this pencil in excess of 82° (twice the critical 

 angle) is reflected back from the cover-glass when a drj'- objective is 

 used, but admitted with an immersion glass, and so follows inevitably 

 the supposition that the value of an immersion lens is simply that it 

 ' obliterates the critical angle for glass,' and so on. 



" When once it is recognized, however, that the large air angle of 

 Fig. 34 and the small balsam angle of Fig. 35 are optically equiva- 

 lent, the whole difficulty vanishes. It is seen that there is no reduc- 

 tion of aperture with balsam-mounted objects (as was so long con-, 

 tended), and that in dealing with the balsam pencil of 170° shown by 

 the dotted lines in Fig. 35, we have been dealing with one not merely 

 equivalent to, but very largely exceeding the air pencil of 170° in Fig. 34. 



* This should strictly be slightly more to represent exactly the 170° in air. 



