( 327 ) 



XXIV.— On a New Property of the Retina. By Sir David Beewster, K.H., 



D.C.L., F.R.S., &c. 



(Read 19th February 1866.) 



In a paper on Hemiopsy, published in the present volume of the Transactions 

 (p. 15), I have mentioned the remarkable fact, that the parts of the retina which 

 are insensible to visual, are sensible to luminous impressions, the light being 

 occasioned by irradiation from the adjacent parts of the retina. The parts thus 

 affected in hemiopsy extend irregularly from the foramen centrale to the margin 

 of the retina ; but the space which they occupy is so small, their distribution so 

 irregular, and the time of their continuance so short, that it is difficult to make 

 such observations upon them as would establish a general property of the 

 retina. 



Mr Airy, our distinguished Astronomer-Royal, who has had more than 

 twenty attacks of hemiopsy, has been induced, by the perusal of my paper, to 

 describe their character, and delineate the form of the parts 

 insensible to visual impressions.* The hemiopsy, in his 

 case, commences at the foramen centrale c, Fig. 1, and ex- 

 tends outwards in a zig-zag curve line, the curve "being 

 small at first, and gradually increasing in dimensions,''' as 

 shown in the figure. It is accompanied with "tremor 

 and boiling so oppressive, that if produced only in one 

 eye, they may nearly extinguish the corresponding vision 

 in the other," and it lasts from twenty to thirty minutes. IriTT 



It occurs sometimes on one side, and sometimes on the 

 other side, of the foramen ; and Mr Airy has " never been able to decide with 

 certainty whether the disease really affects both eyes." On one occasion, when 

 under its influence, he lost " his usual command of speech, and his memory failed 

 so much that he did not know what he had said, or had attempted to say, and 

 that he might be talking incoherently." He, therefore, entertained "no doubt 

 that the seat of the disease was in the brain ; that the disease is a species of 

 paralysis; and that the ocular affection is only a secondary symptom." 



From these important facts, it will be seen that Mr Airy's case differs essen- 

 tially from mine, in which the locality of the indistinctness occurs in irregular 



* Philosophical Magazine, July 1865, vol. xxx. p. 19. 

 VOL. XXIV. PART II. 4 T 



