( 115 ) 



V. — The Uefr active Powers of Peculiar Objectives. 



By Egbert B. Tolles (U.S.). 



In my brief note of December last (1871), communicated to your 

 Journal, I described the action of an immersion objective of 100", 

 constituted of a dry objective of that angle at " uncovered " and of 

 additionally two hemispherical lenses in front, and having the 

 objects in liquid or in cement between, I now propose to borrow 

 in part Mr. AVenham's diagram,* for further illustration. This 

 diagram, Mr. Wenham says, " shows the marginal rays of mean 

 refraction projected in the exact position that they occupy through 

 the ^th described." The extreme aperture is 130°. 



Fig. 1 annexed, except the dotted lines, is a copy of Mr. Wen- 

 ham's as to the front lens and projected rays, with the addition of 

 a double-hemispherical front system, combining the functions of 

 both the immersion front and the balsam-mounted object-slide. It 

 will be noticed that the rays of the |th objective, as traced by Mr. 

 Wenham, meet at the centre of this interposed sphere, because their 

 paths are coincident with its radii. For the same reason this now 

 iaimersion ifh must be of the same angle as the dry |th described 

 by him, viz. (about) 130^. Or to state the case in the words of 

 Mr. Wenham, expository of a surely coincident case, the object 

 being in balsam, " then of course it " (the object) " can be illumi- 

 nated from all angles, and seen by the full aperture of an object- 

 glass." f 



So far there is evident agreement. But the illustration can 

 teach more than this. With balsam between the hemispheres, as 

 before, the object will be traversed and illuminated by a pencil of 

 nearly maximum angle. 



The whole pencil may or may not be passed through the ob- 

 jective used according to its construction. But if air be between 

 the hemispheres and the plane surfaces dusted with minute objects, 

 does anyone imagine large angular pencil to reach the object in this 

 case ? Either from above or below ? Certainly not anyone. The 

 objective being thus used, dry, Mr. Wenham's defiance % might be 

 very safely tendered. 



When air is between the hemispheres, the immersion |th is 

 limited in its angle to the capacity of tlie dry Ith in., when that is 

 used for halsam-mounted objects. That is to say, the first interior 

 reflecting surface (air outside) that the rays meet in any case limits 

 the angle to 82^ ( ~ )? according to the law of total reflexion at 

 interior surfaces. But with any fluid of higher refraction than air 

 between the hemispheres, of course the interior angle of the pencil 



* Found at p. 19, No. xxv., 'Mon Mic. Jour.' 

 t See 'Mon. Mic. Jour.,' No. xxxvi., p. 292. % Ibid., No. xxvii., p. 118. 



