ON LIGHT. . 341 



markable discovery which introduced the term Polariz- 

 ation into optical language. "We find," says he, "that 

 light acquires properties which are relative only to the 

 sides of the ray which are the same for the north and 

 south sides of the ray" [i.e., of a vei-tical ray), " using the 

 points of the compass for description's sake only, and 

 which are different when we oro from the north and south 

 to the east and west sides of the ray." The polarization 

 of light has in fact been an integral part of the science of 

 optics (wanting only a name to designate it) ever since 

 this suggestion of Newton, who derived it from tlie con- 

 templation of one of what Bacon calls *' instantias luci- 

 ferce," luniinifcrous instances^ exliibiting the property or 

 "nature searched after" "in an eminent manner," or in 

 its clearest or most manifest form ; and who described 

 with the utmost clearness and piecision the phaenomenon 

 in which its manifestation consisted in the special case 

 before him.* We shall, therefore, approach the subject 

 from Newton's point of view, choosing for our illustration 

 the very phrenomenon which led him to the singular 

 conclusion embodied in his query. 



* The same phoenomenon is described, and witli no less clear- 

 ness and piecision, by Huyghens, in his admirable work, "Trait^ 

 de la lumiere," pubdished in 1690, fourteen ytai's before the OptxCd 

 of Newton and from that epoch, or from 1678, when that treatise 

 was communicated to the French Academy, must date the discovery 

 of the polarization of light as a fact. Huyghens, moreover, coi'rcclly 

 attributed it to a p^^cuharity impressed on the vibrations of the 

 ethereal medium. But the picturesque phi-ase of Newton embodies 

 the idea in a form easily apprehended, while it seems to have floated 

 rather vaguely before the mind of his great predecessor, not so much 

 as a general attribute, but as a specialty limited to the case in 

 question. 



