CREATION BY LAW. 275 



of heat emitted by all incandescent bodies, — 

 rays which, though intensely hot, are altogether 

 insensible to the eye. Now the Retina of the 

 eye is a piece of mechanism whose Correla- 

 tions are essentially External. It is the ex- 

 pansion of a special nerve whose function it is to 

 be sensitive to certain particular vibrations, and 

 to no other vibrations whatever. Light itself, 

 therefore, is discovered to be merely a relative 

 term — a word, in short, denoting nothing but 

 an external Correlation between the Retina and 

 vibrations of a certain kind and quality. Now 

 what is the language which Professor Tyndall 

 is constrained to use in explanation of facts so 

 difficult of conception ? It is the language of 

 Mechanism, of mental Purpose and Design. " It 

 is not," he says, " the size of a wave which deter- 

 mines its power of producing light ; it is, broadly 

 speaking, the fitness of the wave to the Retina. The 

 ethereal pulses must follow each other with a cer- 

 tain rapidity of succession before they can produce 

 light, and if their rapidity exceed a certain limit, 

 they also fail to produce light. The Retina is 

 attuned, if I may use the term, to a certain range of 



