HERMANN VON HELMHOLTZ 



The other aspect of the question, why the parts of 

 the retina, even when illuminated, as in the eyes of 

 animals having a tapetum, and, in albinos, could not 

 be distinguished, has been made the subject of much 

 discussion. The solution was easy. In the beginning 

 of the eighteenth century, Mery (1704) had observed 

 that he could see the vessels of the retina of a cat 

 immersed in water, the eyes of which were strongly 

 luminous. La Hire, in 1709, gave a correct explana- 

 tion of this phenomenon. He said that it was due to 

 a change in the refraction of the rays that made the 

 eye luminous, but he did not attempt to give a more 

 precise explanation. It was the same with Kussmaul 

 in 1845. He showed that the retina became clear 

 and recognisable when the cornea and the crystallin 

 were removed, or when a small portion of the 

 vitreous was removed, thus shortening the axis of 

 the eye. 



Helmholtz was the first to give a complete account 

 of the relation existing between the directions of the 

 incident and emergent rays, and he gave the true 

 explanation of the blackness of the pupil. He em- 

 ployed for illumination, plane unsilvered glasses, and, 

 to see the retina better, concave lenses. H. Reute 

 (in 1852), on the other hand, was the first to use a 

 mirror having a hole in the centre, and convex glasses. 

 As the new instrument soon acquired great importance 

 in ophthalmology, many ophthalmoscopes of different 

 forms have been made, but they do not involve any 

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