240 RECENT PROGRESS OF THE THEORY OF VISION. 



which follows. Indeed this conception will vary according to 

 circumstances. If we touch the table with two fingers, and 

 feel under each a grain of sand, we suppose that there are 

 two separate grains of sand ; but if we place the two fingers 

 one against the other, and a grain of sand between them, 

 we may have the same sensations of touch in the same two 

 nerves as before, and yet, under these circumstances, we suppose 

 that there is only a single grain. In this case, our consciousness 

 of the position of the fingers has obviously an influence upon 

 the result at which the mind arrives. This is further proved by 

 the experiment of crossing two fingers one over the other and 

 putting a marble between them, when the single object will pro- 

 duce in the mind the conception of two. 



What, then, is it which comes to help the anatomical dis- 

 tinction in locality between the different sensitive nerves, and 

 in cases like those I have mentioned, produces the notion of 

 separation in space ? In attempting to answer this question, 

 we cannot avoid a controversy which has not yet been decided. 



Some physiologists, following the lead of Johannes Miiller, 

 would answer that the retina or skin, being itself an organ 

 which is extended in space, receives impressions which carry 

 with them this quality of extension in space ; that this concep- 

 tion of locality is innate ; and that impressions derived from 

 external objects are transmitted of themselves to corresponding 

 local positions in the image produced in the sensitive organ. 

 We may describe this as the Innate or Intuitive Theory of con- 

 ceptions of Space. It obviously cuts short all further inquiry 

 into the origin of these conceptions, since it regards them as 

 something original, inborn, and incapable of further explana- 

 tion. 



The opposing view was put forth in a more general form by 

 the early English philosophers of the sensational school by 

 Molyneux, 1 Locke, and Jurin. 2 Its application to special 



1 William Molyneux, author of Dioptrica Nova, was born in Dublin, 1656, 

 and died in the same city. 1698. 



2 James Jurin, M.D., Sec. R. S., physician to Guy's Hospital, and President 



