328 



SUMMARY OP CURRENT RESEARCHES RELATING TO 



propose to add rackwork for tlie swinging motion, to obviate tlie 

 uncertainty of moving it by hand. 



Exner's Micro-refractometer.* — If a card is introduced between 

 the eye and the eye-piece of the Microscope and moved towards the 

 axis of the instrument, a point will be reached at which the field is 

 partially darkened, while the objects stand out in relief with sharply 

 defined lights and shadows as in oblique illumination. A transparent 

 object will be gradually obliterated from one side or the other as the 

 card is inserted. If the transparent object is thicker in its centre than 

 at the edges, then if it is also more strongly refracting than the 

 medium by which it is surrounded, the side which is apparently 

 opposite to the card will be the first to become dark ; if, on the other 

 hand, the object is less strongly refracting than the surrounding 

 medium, it will be darkened first on the side from which the screen 

 is introduced. 



The matter will be better understood from fig. 65, ^heie F is the 



Fig. 65. 



objective, C the eye-piece, A the eye, c the object. The lines a h 

 mark the normal course of parallel rays through a non-refracting 

 object, while the lines de represent rays passing through an object 

 which is thicker in the centre than at the edges, and more highly 

 refracting than the surrounding medium. In this case it will be seen 

 that owing to its refraction towards the axis of the instrument the 

 ray e is the first to be obliterated by the screen S when introduced 

 from right to left. With a less highly refracting object, the opposite 

 side will be first obliterated. The dotted lines which diverge from 

 the central point of the object c indicate the effect of oblique illumina- 

 tion, produced when an opaque object is illuminated from above. 

 S should be at the point above the eye-piece to which the rays 

 converge. 



Prof. S. Exner has devised an apparatus (figs. 66 and 67), founded on 

 this principle, which consists of a box fitting by a spring tube above 

 the eye-piece O, with an opening at A, and containing a screen F, with 

 screw motions B and C, by which it can be shifted laterally and raised 

 or depressed. The box can be rotated round the axis of the Micro- 

 scope on the ring r r, and to allow of its being readily removed from 

 the eye-piece, it turns on a pin z, and has a spring catch at E, It 



* Arch. f. Mikr. Anat., xxv. (1885) pp. 97-112 (2 figs, and 1 pi.). 



