SENSE OF VISION SUBJECTIVE PHENOMENA. 777 



black spots be made upon a piece of paper, about four or five inches apart ; 

 then let the left eye be closed, and the right eye be strongly fixed upon the 

 left -hand spot ; if the paper be then moved backwards and forwards, so as 

 to change its distance from the eye, a point will be found at which the right- 

 hand spot is no longer visible, though it is clearly seen when the paper is 

 brought nearer or removed further. In this position of the eye and the 

 object, the rays from the right-hand spot cross to the nasal side of the globe, 

 and fall upon the point of the retina which has just been mentioned. If the 

 same experiment be tried with candles, the image will not entirely disappear, 

 but will become a cloudy mass of light. It is not correct to say, as is some- 

 times done, that the retina is not impressible by light at this point; since, 

 if such were the case, we should see a dark spot in our field of view when- 

 ever we use only one eye, which is not the case. The fact seems rather to 

 be, that this portion of the retina is very inferior to the surrounding parts 

 in its receptivity for luminous impressions; whilst, on the other hand, there 

 is an unusual tendency to the extension of their condition to it by "irradia- 

 tion " ( 633) ; so that, in the experiment just described, if the black spots 

 be made upon a colored ground, this color will take the place of the spot 

 which disappears. 1 



635. That an electrical current is produced when light is allowed to fall 

 upon the Retina, was first established by Holmgren. 2 He observed an os- 

 cillation of the needle of the galvanometer under these circumstances in all 

 vertebrates except Fishes. In the Frog the oscillation is positive, but in all 

 others it is negative, but positive when the light is withdrawn ; the yellow 

 rays are the most active in this respect. The subject has been also taken up 

 by Dewar and McKendrick, 3 who have corroborated and extended Holm- 

 gren's observations. The impression made by rays of light upon the Retina 

 may be to a certain extent imitated by other physical agencies, which thus 

 give rise to various subjective visual phenomena. Advantage has recently 

 been taken by Dr. Serre, 4 of the power of mechanical pressure to produce 

 luminous spectra, for the investigation of the "law of visual direction" 

 ( 620) ; and the results which he has obtained are of very great interest. 

 When any part of the globe of the eye is compressed (the experimenter being 

 in a completely darkened room), a luminous figure is seen to be projected in 

 the direction opposite to the spot pressed upon. Its form varies according 

 to that of the compressing body, and to the degree in which the retina is 

 affected by it. Thus if the pressure be made by the point of the finger, or 

 by any other circular surface, upon a part of the globe over the interior of 

 which the retina is continuous, the spectrum or phosphene (as it is termed by 

 Dr. Serre), is also circular; if the compressing body, on the other hand, be 

 square at its extremity, the " phosphene " is also square ; and if it be trian- 

 gular, the " phospheue " is triangular too. But if the pressure be made near 

 the anterior edge of the retina (which is what most commonly happens, un- 

 less the most favorable situation be designedly chosen), the figure of the 

 " phosphene " is incomplete ; and the degree of its deficiency corresponds with 

 the proportion of the area of compression that does not lie over the retinal 

 expansion. Hence there can be no hesitation in regarding the production 

 of this spectrum as the immediate result of the affection of the sensorium by 



1 The diameter of the blind spot as obtained by experiment on the living eye is 

 1.55mm. (Listing); 1.616mm. (Hannover and Thomsen) ; 181mm. (Helmholtz). 

 The diameter of the optic disk, according to Weber, in the dead subject, varies from 

 2.10 mm. to 1.72 mm. 



2 See Abstract in Humphry and Turner's Journal of Annt. and Phys. , 1872, p. 225. 

 8 See Humphry and Turner's Journal of Anat. and Phys., 1873, vol. vii, p. 275. 



4 See his Essai sur les Phosphfcnes. Paris, 1853. 



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