364 SEA-SIDE STUDIES. 



the rods and cones is one to whicli nervous tissue is 

 confessedly incompetent — namely, tlie transformation 

 of the wave of light into that molecular process 

 which takes place in the conduction of the impres- 

 sion — we may readily admit that their structure is 

 different. 



From what has been already said, it will be easy 

 to i^rove that no images can be formed on the surface 

 of the retina. In the first place, the retina, during 

 life, is as transparent as glass. The rays of light must 

 therefore pass through it, and enter the pigment layer, 

 which, being perfectly black, absorbs all rays. Fur- 

 ther, it has been proved that the optic fibres are 

 totally insensible to light. There is a blind spot in 

 each eye. Would you know the peculiarity of that 

 spot 1 It is where the optic nerve enters, and where, 

 consequently, nothing but nerve-fibres exist. There is 

 also a spot in each eye where the sensitiveness to light 

 is at its maximum. Would you know the peculiarity 

 of that spot? It is a mass of cells, without a con- 

 tinuous surface-layer of fibres. After jDroving that 

 the fibres are insensible to lio-ht, and that no imao-e is 

 formed where the fibres alone exist, we are called 

 upon to show that some apparatus exists for the 

 reception of these rays of light out of which the neces- 

 sary images are formed ; and to Professor Draper we 

 must turn for the best hypothesis to aid us. 



Franklin, he reminds us, placed variously-coloured 

 pieces of cloth in the sunlight on the snow. They were 

 so arranged that the rays should faU on them equally. 



