Chapter V 



OPHTHALMOSCOPY, OR THE EXAMINATION THROUGH THE 



PUPILS OF ANIMATE SPECIMENS OF THE INTERIOR 



OF THE VERTEBRATE EYE BY MEANS 



OF THE OPHTHALMOSCOPE 



Since this subject, although familiar to 

 ophthalmologists (oculists), is rarely under- 

 stood by naturalists in general, and espe- 

 cially not by ornithologists, a brief review of 

 ophthalmoscopy as practiced on the verte- 

 brate eye may not be out of place in a work 

 like this. 



The ancients noticed that the eyes of some 

 animals are, under certain conditions, bril- 

 liant in twilight or semi-darkness. Although 

 he did not attempt to explain the phenome- 

 non, Pliny remarks that the eyes of animals 

 that see at night — cats, for example — are 

 radiant and shining, and that the eyes of 

 the she-goat and the wolf emit a light like 

 fire. This fact continued to be noted through 

 the succeeding centuries and was commented 

 on more or less by other observers. Jean 

 Mery of Paris, who in 1704 immersed a cat in 

 water, beheld in all its glory the fundus of 

 that animal's eye, including the optic nerve 

 entrance, the retinal blood vessels, and the 

 remarkable coloration of the retina and cho- 

 roid. Mery did not give a proper explana- 

 tion of this phenomenon, but five years later 

 de la Hire showed that the refractive power 

 of the cornea is neutralized by immersion of 

 the animal's eyes in water, so that all rays of 

 light reflected from a given point of the fundus 

 emerge from the pupil not as parallel but as 

 divergent rays; consequently the fundus im- 

 ages can be seen as if one were looking at them 

 in the depths of the eye through air. 



The Fundus Oculi or Background of the 

 Internal Eye in Man. Glimpses of the human 



ocular interior were obtained from time to 

 time but it was not until the year 1847, when 

 Babbage, an English mathematician, exhibited 

 to Wharton Jones, a well-known oculist of 

 his day, the model of an instrument by 

 means of which the interior of the eye could 

 be thoroughly examined. It consisted of 

 a small, plane, glass mirror from whose centre 

 a portion of the silvering had been removed. 

 This device was not made known to the world 

 until 1854, before which date the celebrated 

 physicist, Hermann von Helmholtz, published, 

 in 1851, a description of an ophthalmoscope 

 which enables the observer to see with ease 

 all the parts in the background of the eye. 



The Ophthalmoscope. The original Helm- 

 holtz instrument consisted of four thin plates 

 of glass carefully polished, screwed together, 

 fastened at an angle of fifty-six degrees to a 

 brass disk, and forming the hypothenuse of 

 a right-angled triangular prism. The other 

 sides of this hollow prism were made of metal, 

 and all carefully blackened inside. The brass 

 disk had a hole in its centre and an arrange- 

 ment to place a concave lens over it; for 

 normal eyes Helmholtz used a No. 10 (4 d.) 

 glass. The light from a lamp falling on the 

 glass plates was in part reflected into the 

 observed eye, while the observer, looking 

 through the concave lens and the opening in 

 the brass disk, received the returning rays in 

 his own eye, and was able to see the fundus, 

 weakly illuminated, but still distinctly. 



The instrument noio in use for examining 

 the interior of the eye more nearly resembles 



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