242 PSYCHOLOGY. 



many cases — for example, in the phenomena of the blind spot, in the 

 discrimination of over-tones and combination-tones from the ground- 

 tone of musical sounds, etc. — such a strain of the attention is required, 

 even with appropriate instrumental aids, that most persons fail. The 

 very after-images of bright objects are by most men perceived only 

 under exceptionally favorable conditions, and it takes steady practice 

 to see the fainter images of this kind. It is a commonly recurring ex- 

 perience that persons smitten with some eye-disease which impairs 

 vision suddenly remark for the first time the muscce voUtantes which 

 all through life their vitreous humor has contained, but which they now 

 firmly believe to have arisen since their malady ; the truth being that 

 the latter has only made them more observant of all their visual sensa- 

 tions. There are also cases where one eye has gradually grown blind, 

 and the patient lived for an indefinite time without knowing it, imtil, 

 through the accidental closure of the healthy eye alone, the blindness 

 of the other was brought to attention. 



"Most people, when first made aware of binocular double images, 

 are uncommonly astonished that they should never have noticed them 

 before, although all through their life they had been in the habit of see- 

 ing singly only those few objects which were about equally distant with 

 the point of fixation, and the rest, those nearer and farther, which con- 

 stitute the great majority, had always been double. 



"We must then learn to turn our attention to our particular sensa- 

 tions, and we learn this commonly only for such sensations as are means 

 of cognition of the outer world. Only so far as they serve this end have 

 our sensations any importance for us in ordinary life. Subjective 

 feelings are mostly interesting only to scientific investigators ; were 

 they remarked in the ordinary use of the senses, they could only cause 

 disturbance. Whilst, therefore, we reach an extraordinary degree of 

 firmness and security in objective observation, we not only do not reach 

 this where subjective phenomena are concerned, but we actually attain 

 in a high degree the faculty of overlooking these altogether, and keep- 

 ing ourselves independent of their influence in judging of objects, even 

 in cases where their strength might lead them easily to attract our at- 

 tention." (Physiol. Optik, pp. 431-2.) 



Even where the sensation is not merely subjective, as in 

 the cases of which Hehnholtz speaks, but is a sign of some- 

 thing outward, we are also liable, as Reid says, to overlook 

 its intrinsic quality and attend exclusively to the image of 

 the ' thing ' it suggests. But here everyone caw easily notice 

 the sensation itself if he wilL Usually we see a sheet of 

 paper as uniformly white, although a part of it may be in 

 shadow. But we can in an instant, if we please, notice the 

 shadow as local color. A man walking towards us does 

 not usually seem to alter his size ; but w^e can, by setting 



