MKMOIK VI.] HYPOTHESES OF VISION. 101 



I may also refer to my Treatise on "Human Physiology" 

 (1856). 



There are many rays emitted by the sun and other 

 shining bodies to which our eyes are entirely blind. 



Two different reasons may be alleged for our inability 

 to perceive such rays : first, they may not be able to reach 

 the retina, the media of the eye not transmitting them ; 

 second, the retina may be so constituted as to be unable 

 to receive their impressions. 



It has long been known that rays which come from 

 sheet-iron heated by a lamp cannot pass either through 

 the cornea or through the crystalline lens. Even of 

 those that are furnished by an Argand flame, used as a 

 luminous source of heat, less than one fifth pass through 

 the cornea alone, and scarcely one fiftieth when the crys- 

 talline lens is interposed. Cima showed that of the heat- 

 rays emitted by a flame, less than one tenth pass through 

 the cornea, lens, and vitreous humor conjointly. Janssen, 

 using a flame, compared the heat transparency of the 

 separate media of the eye with that of water included 

 between glass plates, showing that there is a perfect 

 accordance between them if taken of equal thickness. 

 From this it is to be concluded that invisible rays to a 

 certain extent reach the retina. Franz, by carefully con- 

 ducted experiments with a thermo-electric pile, came to 

 the conclusion that a quantity of obscure rays detectable 

 by the thermometer can reach the retina, which there- 

 fore must be so constituted as not to be able to per- 

 ceive them. 



This settles the question so far as the less refrangible 

 or ultra-red rays are concerned. We have then to de- 

 termine how it is with those at the opposite or more re- 

 frangible end of the spectrum. Do these pass through 

 the media of the eye, or are they arrested and never 

 reach the retina? 



