939 



image prism of calcspar and glass is placed between the condensor 

 and the slit and causes two images of the filament to be projected 

 in the plane of the slit. Only one of these, formed by the ordinary 

 rays falls in the slit, the other falls on one of the slitplates and 

 is arrested. Consequently tin; eye is illuminated with polarised light 

 and the images reflected by the ophthalmoscope lens are also |)olarised. 

 By means of a nicol prism placed in the microscope tube these 

 reflexes are extinguished. The light illuminating the retina and 

 reflected from the fundus of the eye has become depolarised and 

 can be observed with the microscope. As a matter of fact the retina 

 is clearly seen without any appreciable disturbing reflexes from the 

 surface of the intervening media. Also the retinalreflexes, which in 

 young patients are nearly always very noticeable, seem to be 

 very slightly lessened. 



The construction of this ophthalmoscope appears to possess some 

 advantages. According to the Helmholtz-Gullstrand theory we use 

 H)ne small part of the pupil in the patient's eye for transmitting 

 the illuminating light-cone, whereas another part of the pupil takes 

 up the narrowest part of the double cone of rays emerging from 

 the retina and passing into the eye of the observer. These cones 

 should be entirely separated by a narrow unused zone both of 

 the cornea and of the anterior and posterior sui-face of the lens. 

 Only in this way is it possible to prevent the occurrence of reflexes 

 emerging from these surfaces which pass into the eye of the observer 

 and disturb the ophthalmoscopic image by diffused light from the 

 substance of the cornea and the lens. With my instrument the 

 reflexes can never reach the observer as they would also be obscured 

 by the nicol prism. Therefore we have only to consider the light 

 diffused by the illuminated parts of cornea and lens. The lens is in 

 this respect more troublesome than the cornea, especially in young 

 individuals, whereas in adult patients both show nearly the same 

 opalescence. Consequently we might in some cases — at least 

 theoretically — lessen the distance between the illuminating and 

 observation cone of light, and we should be able to examine eyes 

 with narrow pupils — at least smaller ones with our instrument 

 than with other instruments of the same kind. The reason that 1 

 have adjusted the instrument without considering this possibility 

 may be found in the fact that I wished to have an instrument which 

 would be ready for use with any patient without any adjustment 

 except of course the final focussing. 



With this instrument we can see at once 27° of the fundus of an 

 emmetrope eye, corresponding to about 4'/, diameters of the papilla 



