THE PERCEPTION OF SPACE. 241 



iiified, without making the least reflection upon the sign, or even per- 

 ceiving that there is any such thing. . . . The mind has acquired a con- 

 firmed and inveterate habit of inattention to them (the signs). For 

 they no sooner appear than, quick as lightning, the thing signified suc- 

 ceeds and engrosses all our regard. They have no name in language ; 

 and although we are conscious of them when they pass through the 

 mind, yet their passage is so quick and so familiar that it is absolutely 

 unheeded ; nor do they leave any footsteps of themselves, either in the 

 memory or imagination." (Inquiry, chap. v. §§ 2, 3.) 



If we review tlie facts we shall find every grade of non- 

 attentiou between the extreme form of overlooking men- 

 tioned by Reid (or forms even more extreme still) and com- 

 plete conscious perception of the sensation present. Some- 

 times it is literally impossible to become aware of the latter. 

 Sometimes a little artifice or effort easily leads us to discern 

 it together, or in alternation, with the ' object ' it reveals. 

 Sometimes the jiresent 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 sensa- 

 tions, strictly so called, or those which are not signs of 

 outer objects at all. Helmholtz's treatment of these phe- 

 nomena, muscce volitantes, negative after-images, double 

 images, etc., is very satisfactory. He says : 



" We only attend with any ease and exactness to our sensations in so 

 far forth as they can be utilized 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 observation of these latter more subjective feelings. Although it 

 might seem that 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 Purkinje, or acci- 

 dent or theoretic speculation, are necessary conditions for the discovery 

 of subjective phenomena. 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 espe- 

 cially exercised on subjective phenomena to discover this one or that ; 

 only where the subjective appearances are so intense that they inter- 

 fere with the perception 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 



