518 PSYCHOLOGY. 



tive phenomena are extraordinarily hard to find ; and when they are 

 once found, special aids for the attention are almost always required to 

 observe them. It is usually hard to notice the i)henomenon again even 

 when one knows already the description of the first observer. The 

 reason is that we are not only unpractised in singling out these subjec- 

 tive sensations, but tliat we are, on the contrary, most tlioroughly 

 trained in abstracting our attention from them, because they would 

 only hinder us in observing the outer world. Only when their inten- 

 sity is so strong as actually to hinder us in observing the outer world 

 do we begin to notice them ; or they may sometimes, in dreaming and 

 delirium, form the starting point of hallucinations. 



" Let me give a few well-known cases, taken from physiological optics, 

 as examples. Every eye probably contains muscce volitantes, so called ; 

 these are tibres, granules, etc., floating in the vitreous humor, throwing 

 their shadows on the retina, and appearing in the field of vision as 

 little dark moving spots. They are most easily detected by looking at- 

 tentively at a broad, bright, blank surface like the sky. Most persons 

 who have not had their attention expressly called to the existence of 

 these figures are apt to notice them for the first time when some ail- 

 ment befalls their eyes and attracts their attention to the subjective 

 state of these oi'gans. The usual complaint then is that the muscce 

 voUtantes came in with the malady ; and this often makes the patients 

 very anxious about these harmless things, and attentive to all their 

 peculiarities. It is then hard work to make them believe that these 

 figures have existed throughout all their previous life, and that all 

 healthy eyes contain them. I knew an old gentleman who once had 

 occasion to cover one of his eyes which had accidentally become dis- 

 eased, and who was then in no small degree shocked at finding that his 

 other eye was totally blind ; with a sort of blindness, moreover, which 

 must have lasted years, and yet he never was aware of it. 



" Who, besides, would believe without performing the appropriate ex- 

 periments, that when one of his eyes is closed there is a great gap, the so- 

 called ' blind spot,' not far from the middle of the field of the open eye, in 

 which he sees nothing at all, but which he fills out with his imagination ? 

 Mariotte, who was led by theoretic speculations to discover this 

 phenomenon, awakened no small surprise when he showed it at the 

 court of Charles II. of England. The experiment was at that time 

 repeated with many variations, and became a fashionable amusement. 

 The gap is, in fact, so large that seven full moons alongside of each 

 other would not cover its diameter, and that a man's face 6 or 7 feet 

 off disappears within it. In our ordinary use of vision this great hole 

 m the field fails utterly to be noticed ; because our eyes are constantly 

 wandering, and the moment an object interests us we turn them full 

 upon it. So it follows that the object which at any actual moment 

 excites our attention never happens to fall upon this gap, and thus it 

 is that we never grow conscious of the blind spot in the field. In order 

 to notice it, we must first purposely rivet our gaze upon one object and 



