538 VISUAL PERCEPTIONS. [BOOK m. 



nerve is quite insensible to light, we are conscious of no blank in the 

 field of vision. When in looking at a page of print we fix the 

 visual axis so that some of the print must fall on the blind spot, no 

 gap is perceived. We could not expect to see a black patch, be- 

 cause what we call black is the absence of the sensation of light 

 from structures which are sensitive to light ; we must have visual 

 organs to see black. But there are no visual organs in the blind 

 spot, and consequently we are in no way at all affected by the 

 rays of light which fall on it. There is in our subjective field of 

 vision no gap corresponding to the gap in the retinal image. We 

 refer the sensations coming from two points of the retina lying on 

 opposite margins of the blind spot to two points lying close 

 together, since we have no indication of the space which separates 

 them. Concerning the effects which are produced when an object 

 in the field of view passes into the region of the blind spot there 

 has been much discussion. In ordinary vision of course, the exist- 

 ence of the blind spot is of little moment since it is outside the 

 region used for distinct vision, and besides the image of an object 

 does not fall on the blind spots of both eyes at the same time. 



Ocular Spectra. So far from our perceptions exactly corre- 

 sponding to the arrangements of the luminous rays which fall on 

 the retina, we may-have visual sensations and perceptions in the 

 entire absence of light. Any stimulation of the retina or of the 

 optic nerve sufficiently intense will give rise to a visual sensation. 

 Gradual pressure on the eyeball causes a sensation of rings of colour- 

 ed light, the so-called phosphenes ; a sudden blow on the eye causes 

 a sensation of flashes of light, and the seeming identity of the 

 visual sensations so 'brought about with visual sensations produced 

 by light is well illustrated by the statement once gravely made in 

 a German court of law, by a witness who asserted that on a pitch 

 dark night he recognised an assailant by help of the flash of light 

 caused by the assailant's hand coming in violent contact with his 

 eye. Electrical stimulation of the eye or optic nerve will also give 

 rise to visual sensations. 



The sensations which may arise without any light falling on the 

 retina need not necessarily be undefined; on the contrary they 

 may be most clearly defined. Complex and coherent visual images 

 or perceptions may arise in the brain without any corresponding 

 objective luminous cause. These so-called ocular spectra or phan- 

 toms, which are the result of an intrinsic stimulation of some 

 (probably cerebral) part of the visual apparatus, have a distinctness 

 which gives them an apparent objective reality quite as striking as 

 that of ordinary visual perceptions. They may occasionally be 

 seen with the eyes open (and therefore while ordinary visual per- 

 ceptions are being generated) as well as when the eyes are closed. 

 They sometimes become so frequent and obtrusive as to be dis- 



