Page:Mind (Old Series) Volume 12.djvu/364

THE PERCEPTION OF SPACE, (ill.) 351 Sometimes a little artifice or effort easily leads us to discern it together, or in alternation, with the ' object ' it reveals. Sometimes the present sensation is held to be the object or to reproduce its features in undistorted shape, and then, of course, it receives the mind's full glare. The deepest inattention is to subjective optical sensations, strictly so called, or those which are not signs of outer objects at all. Helmholtz's treatment of these phenomena, muscae wlitantes, negative after-images, double images, &c., is very satisfactory. He says : " We only attend with any ease and exactness to our sensations in so far forth as they can be utilised for the knowledge of outward things ; and we are accustomed to neglect all those portions of them which have no significance as regards the external world. So much is this the case that for the most part special artifices and practice are required for the observa- tion of these latter more subjective feelings. Although it might seem nothing should be easier than to be conscious of one's own sensations, experience nevertheless shows that often enough either a special talent like that showed in eminent degree by Purkirije, or accident or theoretic speculation, are necessary conditions for the discovery of subjective pheno- mena. Thus, for example, the blind spot on the retina was discovered by Mariotte by the theoretic way ; similarly by me the existence of 'summation' tones in acoustics. In the majority of cases accident is what first led observers whose attention was especially exercised on subjective phenomena to discover this one or that ; only where the subjective appearances are so intense that they interfere with the per- ception of objects are they noticed by all men alike. But if they have once been discovered it is for the most part easy for subsequent observers who place themselves in proper conditions and bend their attention in the right direction to perceive them. But in many cases for example, in the phenomena of the blind spot, in the discrimination of overtones and combination-tones from the ground-tone of musical sounds, &c. 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 favourable conditions, and it takes steady practice to see the fainter images of this kind. It is a commonly recurring experience that persons smitten with some eye-disease which impairs vision suddenly remark for the first time the muscae volitantes which all through life their vitreous humour 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 sensations. There are also cases where one eye has gradually grown blind, and the patient lived for an indefinite time without knowing it, until, through the accidental closure of the healthy eye alone, the blind- ness 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 seeing singly only those few objects which were about equally distant with the point of fixation, and the rest, those nearer and farther, which constitute the great majority, had always been double. " We must then learn to turn our attention to our particular sensations, and ; we learn this commonly only for such sensations as are means of