250 ANIMAL PHYSIOLOGY. 



can be temporarily changed by pressure on the shut eyes, and 

 will again return. 



186. The effects of pressure on coloured spectra afford an 

 interesting illustration of the fact that mechanical irritation of 

 the retina produces a sense of light, and not of pain. I gaze 

 on the name of the lamp beside me, and on shutting my eyes 

 I see a spectrum, which, instead of being of the comple- 

 mentary colour, is bright yellow, with a red margin, and float- 

 ing on a dark green halo. I press my fingers against my 

 closed eyes, and obtain the complementary colour, namely, a 

 violet spectrum with a green margin on a yellow halo. On 

 removing the pressure the original colours return; and this 

 can be repeated several times. The appearances are even 

 more complex, if the experiment be made with sunlight. 



Phenomena of light uncomplicated with colour can likewise 

 be obtained by mechanical irritation. An accidental blow 

 on the eye produces an appearance of sparks of fire; and by- 

 gentle pressure the effects called phosphenes are obtained. 

 A phosphene is a luminous image produced by shutting the 

 eyes, and touching one of them lightly but firmly on the 

 outer, inner, upper, or lower border in short, on any part 

 where the retina extends. A luminous crescent, or complete 

 circle, flashes into sight at the point diametrically opposite 

 the pressure. This is called the larger phosphene, and 13 

 caused by irritation of the retina at the point touched, re- 

 ferred by the mind, like all retinal impressions, to the posi- 

 tion vertically opposite. Besides this, a smaller pliosphene 

 may be obtained, visible at the part touched, which is caused 

 by the contents of the eyeball being pressed against the 

 opposite point. The smaller phosphene is a blush of light of 

 variable intensity, extending over a space, larger or smaller, 

 according to the size of the object with which pressure is 

 made; the larger phosphene is always brilliant, evanescent, 

 and confined to a ring. Phosphenes are much more easily 

 produced at one time than another ; and after reading to a 

 late hour, the mere closure of the eyelids in a dark room may 

 cause a bright circle of light to flash before each eye. 



By means of pressure, patterns produced by a number of 

 internal structures of the eyeball can. be brought into view. 

 The branches of the retinal artery may thus be seen as dark 



