EPIPOLIC PHENOMENA. 131 



these phenomena, which are included under the term of 

 Fluorescence, is of that interesting and important cha- 

 racter, that it must be ranked as the most decided ad- 

 vance which has been made in physical optics since the 

 days of Newton. 



It is not improbable that those rays of such high 

 refrangibility may, although they are under ordinary 

 circumstances invisible to the human eye, be adapted 

 to produce the necessary degree of excitement upon 

 which vision depends in the optic nerves of the night- 

 roaming animals. The bat, the owl, and the cat, may 

 see in the gloom of night by the aid of rays which are 

 invisible to, or inactive on the eyes of man, or of those 

 animals which require the light of day for perfect vision. 



It is a general law of the radiant forces, that when- 

 ever they fall upon any surface, a portion is thrown 

 back or reflected at the same time as other portions are 

 absorbed or transmitted. Upon this peculiarity appear 

 to depend the phenomena of natural colour in bodies. 



The white light of the sun is well known to be com- 

 posed of several coloured rays. Or rather, according to 

 the theory of undulations, when the rate at which a ray 

 vibrates is altered, a different sensation is produced upon 

 the optic nerve. The analytical examination of this 

 question shows, that to produce a red colour the ray of 

 light must give 37,640 undulations in an inch, and 

 458,000000,000000. in a second. Yellow light requires 

 44,000 undulations in an inch, and 535,000000,000000 

 in a second; whilst the effect of blue results from 5 1,1 10 

 undulations within an inch, and 622,000000,000000 of 

 waves in a second of time.* The -determination of such 



* In connection with this view, the Newtonian theory should 

 be consulted, for which see A Letter of Mr. Isaac Newton, Pro- 

 fessor of the Mathematicks in the University of Cambridge ; con- 

 taining his new Theory about Light and Colors : sent by the Author 

 to the Publisher, from Cambridge, leb, 6, 1671-72, in order to be 

 communicated to the Royal Society. 



