216 PSYCHOLOGY. 



colonel stood still, the result would be the same. In such 

 wajs as these a creature endowed with eyes alone could 

 hardly fail of measuring out all three dimensions of the 

 space he inhabited. And we ourselves, I think, although 

 we may often ' realize ' distance in locomotor terms 

 (as Berkeley says we must always do), yet do so no less 

 often in terms of our retinal map, and always in this way 

 the more spontaneously. Were this not so, the three visual 

 dimensions could not possibly feel to us as homogeneous as 

 they do, nor as commensurable inter se. 



Let us then admit distance to be at least as genuinely optical 

 a content of conscioiisness as eitJwr height or breadth. The 

 question immediately returns, Can any of them be said in any 

 strictn£ss to be optical sensations ? We have contended all 

 along for the affirmative reply to this question, but must 

 now cope with difficulties greater than any that have as- 

 sailed us hitherto. 



Hdmhjoltz and Eeid on Sensations. 



A sensation is, as we have seen in Chapter XVII, 

 the mental affection that follows most immediately upon 

 the stimulation of the sense tract. Its antecedent is di- 

 rectly physical, no psychic links, no acts of memory, infer- 

 ence, or association intervening. Accordingly, if we sup- 

 pose the nexus between neural process in the sense-organ, 

 on the one hand, and conscious affection, on the other, to 

 be by nature uniform, the same process ought always to give 

 the same sensation ; and conversely, if what seems to be a sen- 

 sation varies whilst the process in tJie sense-organ remains un- 

 changed, the reason is presumably that it is really not a sensa- 

 tion but a higher mental product, lohereof the variations depend 

 on events occurring in the system of higher cerebral centres. 



Now the size of the field of view varies enormously in all 

 three dimensions, without our being able to assign with any 

 definiteness the process in the visual tract on which the 

 variation depends. We just saw how impossible such 

 assignment was in the case where turning down the head 

 produces the enlargement. In general, the maximum feel- 

 ing of depth or distance seems to take the lead in deter- 

 mining the apparent magnitude of the whole field, and the 



