ON LIGHT. 221 



not familiar with some of the most intricate departments 

 of mathematical science. In explaining such features 

 (when unavoidable), without prejudice to the strictness 

 of mathematical reasoning adducible and held to be 

 conclusive and satisfactory by those who have mastered 

 it, we must have recourse to analogies more or less close 

 with processes we see going on in nature ; and which, 

 whether perfectly understood or not in their modus opcr- 

 andi, we, at all events, perceive to consist in a sequence 

 of events, comprehensible in themselves and arising 

 naturally and familiarly one out of another. There are 

 many phenomena of polarized light which admit of be- 

 ing so, as it were, shadowed forth to the mind of a 

 beginner as analogous to things familiar enough. In 

 such cases, though the analogy may be imperfect, or 

 even altogether incompetent to stand for an explana- 

 tion, the phenomenon is sometimes so neatly conveyed 

 to the intellect, that by generalizing to the extreme all 

 the terms used in describing the one, it is very conceiv- 

 able that the cardinal feature of the other that which 

 dominates its whole explanation may be included. 

 Even if not so, the object is so far answered, that the 

 student remains possessed of a mental picture which 

 will not allow him to forget its prototype. And it is 

 not a compendium of Optics, or an essay on Vision, or 

 an account of telescopes, microscopes, or other optical 

 instruments, that he has here to expect. Nothing of the 

 kind could by possibility be comprised within such limits 

 as a contributor to a work of this kind must necessarily 

 observe. Suffice it to convey to his apprehension some 



