VI 



DIOPTRIC MECHANISM OF THE EYE 



323 



and in man. This problem was solved a century and a half later 

 by Helmholtz (1851), after some preliminary attempts by Briicke. 

 Briicke noted that the human eye which has no tapetuin can 

 also, under certain conditions, reflect enough light to make the 

 field of the pupil appear red. The eye must be illuminated in a 

 dark room by a light held sideways to the observer's eye, and 

 the observed eye must not be accommodated to the light. The 



FIG. 146. Diagram to show Briicke'.s method of seeing the illuminated fundus of the eye. 



diagram (Fig. 146) shows the arrangement of the experiment. 



As the observed eye A is not accommodated to the luminous 



point L, a diffusion-circle ab is formed on the retina ; the light is 



reflected from this diffusion-circle outwards in the form of a 



cone cKd, which has its nodal point at".Z The observing eye B, 



which is near L and protected from the direct rays of light and 



heat by the screen M, is in the field 



of the cone of divergent rays issuing 



from A, and the pupil of the latter, 



therefore, appears red. It is obvious 



that the observed eye must be hyper- 



metropic or myopic if the observer is 



to see it illuminated ; an emmetropic 



(accommodated) eye which reflects 



the rays from the fundus in a parallel 



direction cannot be seen illuminated, 



because the observer is unable to 



bring his eye into the field of the 



reflected rays without intercepting 



the source of light with his head. 



According to Briicke's observa- 

 tions, it was necessary, in order to see 

 the fundus of an eye of any refractive 

 ]niwer, to find a method of bringing the eye of the observer into 

 the path of the rays reflected from the observed eye without inter- 

 cepting the light that made it visible. Helmholtz succeeded in 

 doing this in 1852. He illuminated the observed eye by light 

 reflected from three superposed glass plates, through which he was 

 able, owing to their perfect transparency, to see the fundus illumin- 

 ated red, no matter what the refractive power of the eye, provided 

 the pupil was sufficiently dilated to admit of adequate illumination. 



A ft 



3 



A^ f 



Fn:. 147. Diagram to show Helmholtz' 

 method of seeing the illuminated fundus 

 of the eye. 



