780 SIGHT. 



discussion. In ordinary vision, of course, the existence 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. [See Fig. 179.] 



678. Ocular spectra. So far from our perceptions exactly correspond- 

 ing 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 colored 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 recog- 

 nized 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 phantoms, which are the result of an intrinsic stimu- 

 lation of some (probably cerebral) part of the visual apparatus, have a dis- 

 tinctness 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 perceptions are being 

 generated) as well as when the eyes are closed. They sometimes become so 

 frequent and obtrusive as to be distressing, and form an important element 

 in some kinds of delirium, such as delirium tremens. 



679. Appreciation of apparent size. By the eye alone we can only esti- 

 mate the apparent size of an object, we can only tell what space it takes in 

 the field of vision, we can only perceive the dimensions of the retinal image, 

 and therefore have a right only to speak of the angle which the diameter of 

 the object subtends. The real size of an object must be determined by other 

 means. But our perception of even the apparent size of an object is so 

 modified by concurrent circumstances that in many cases it cannot be relied 

 on. The apparent size of the moon must be the same to every eye, and yet 

 while some persons will be found ready to compare the moon in the mid- 

 heavens with a three-penny piece, others will liken it to a cart-wheel ; that 



FIG. 178. 



is to say, the angle subtended by the moon seems to the one to be about 

 equal to that subtended by a three-penny piece held at the distance from the 

 eye at which it is most commonly looked at, and to the other about equal to 

 that subtended by a cart-wheel similarly viewed at the distance at which it 

 is most commonly looked at. If a line such as A C, Fig. 178, be divided 

 into two equal parts, A B, B C\ and A B be divided by distinct marks into 

 several parts, as is shown in the figure, while B C is left entire, the distance 

 A B will always appear greater than CB. So also, if two equal squares be 

 marked, one with horizontal and the other with vertical alternate dark and 

 light bands, the former will appear higher, and the latter broader, than it 

 really is. Hence short persons affect dresses horizontally striped in order to 



