152 ANALOGY OF SOUND AND LIGHT. 



sensations of pain or pleasure may result, in the same 

 way as sharp or subdued sounds are disagreeable or 

 otherwise. Intensely coloured bodies do make an im- 

 pression upon perfectly blind men; and those who, 

 being born blind, know no condition of light or colour, 

 will point out a difference between strongly illuminated 

 red and yellow media. When the eyes are closed we 

 are sensible to luminous influence, and even to differ- 

 ences of colour. We must consequently infer that light 

 produces some peculiar action upon the system of nerves 

 in general ; this may or may not be independent of the 

 chemical agency of the solar radiations ; but certainly 

 the excitement is not owing to any calorific influence. 

 The system of nerves in the eye is more delicately 

 organized, and of course peculiarly adapted to all the 

 necessities of vision. 



Thus far some analogy does appear to exist between 

 light and sound ; but the phenomena of the one are so 

 much more refined than those of the other the 

 impressions being all of them of a far more complicated 

 character, that we must not be led too far by the analo- 

 gical evidence in referring light, like sound, to mere 

 material motion. 



It was a beautiful idea that real impressions of ex- 

 ternal objects are made upon the seat of vision, and that 

 they are viewed, as in a picture, by something behind 

 the screen, that these pictures become dormant, but 

 are capable of being revived by the operations of the 

 mind in peculiar conditions ; but we can only regard it 

 as a philosophical speculation of a poetic character, the 

 truth or falsehood of which we are never likely to be 

 enabled to establish.* 



* " I would here observe that a consideration of many such 

 phenomena (the obliteration and revival of photographic drawings) 

 lias led me to regard it as not impossible that the retina itself may 

 be photographically impressed by strong light, and that some at 

 least of the phenomena of visual spectra and secondary colours 



