THEORIES OF LIGHT. 



These are the two, and the only two modes, in which it was ever 

 imagined that a distant object could become visible to the eye. 



2. In the first, there is an analogy between the eye and the 

 organs of smelling. Odorous objects do actually emit material 

 effluvia, which form part of their own substance. These effluvia 

 reach the organ of smelling, and produce upon it a specific effect, 

 which impresses the mind with a corresponding perception. 

 According to the first supposition, a visible object at any distance 

 would act in the same way, and would eject continual particles 

 of light, which particles of light would move to the eye and 

 produce vision, acting mechanically on its membrane in the 

 same manner as the effluvia of a rose produce a sensible eifect 

 upon the organs of smelling. 



3. The second method places the eye in analogy with the ear. 

 So close is this analogy, that all the mathematical formulae by 

 which the effects of sound are expressed in acoustics, will, with 

 very slight changes, be capable of expressing the effects of vision, 

 according to the latter hypothesis. 



4. It is evident, however, that as the first hypothesis requires 

 us to admit that distant visible objects are continually ejecting 

 matter from their surfaces to produce vision ; so the second 

 hypothesis as peremptorily requires the admission of the existence 

 of some physical medium pervading the universe, some subtle 

 ethereal fluid endowed with a property of propagating the pulsa- 

 tions or undulations of distant visible objects, and transmitting 

 them to the eye. This hypothetical fluid has been called the 

 luminiferous ether. 



5. The first of these two celebrated theories of light has been 

 called the CORPUSCULAR THEORY, and the second the UNDULATORY 

 THEORY. 



Newton, although he did not identify his investigations in 

 optics with any hypothesis, but in the spirit of the inductive 

 philosophy founded by Bacon based his conclusions on experi- 

 ments and observations only, adopted nevertheless the nomen- 

 clature and language of the corpuscular theory, and, probably, 

 from veneration for his authority, English philosophers, until 

 recently, have very generally given the preference to that 

 theory. 



The undulatory theory, on the other hand, was adopted by 

 Huygens, and after him by most continental philosophers. 



Optical researches within the last hundred years have been 

 prosecuted with singular diligence and success. A vast variety 

 of phenomena previously unknown, have been accurately investi- 

 gated, new laws have been developed, and the general result has 

 been that the undulatory theory has prevailed over the corpus- 



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