224 



LIGHT. 



We answer that there are two and only two ways in which it is possible to 

 conceive such an action to take place. These two are the following : 



First. The distant object thus visible to us, may emit particles of matter 

 from its surface, which particles of matter may pass over the intervening 

 space, may enter the pupil of the eye, may strike upon the nervous mem- 

 brane, arid so affect it as to produce vision. 



Secondly. There may be in the space between the distant visible object 

 and the eye, a medium possessing elasticity, so as to be capable of receiving 

 and transmitting pulsations or undulations like those imparted to the air by a 

 sounding body. If this be admitted, the distant visible object may, without 

 emitting any particles of matter from its surface affect such a medium sur- 

 rounding it with pulsations or undulations, in the same manner as a bell 

 affects the air around it. These pulsations or undulations may pass along the 

 space intervening between the visible object and the eye, in the same manner 

 as the pulsations or undulations produced by a bell pass along the air between 

 the bell and the ear. In this manner the pulsations transmitted from the 

 visible object, and propagated by the medium, we have referred to, may reach 

 the eye and affect the membrane which lines it, in the same manner ex- 

 actly as the pulsations in the air affect the tympanum of the ear. 



These are the two, and the only two modes, in which any human mind ever 

 yet conceived that a distant object could become visible to the eye. 



In the first, there is an analogy between the eye and the organs of smelling. 

 Odorous objects do actually emit material effluvia, which must be supposed to 

 form part of their own substance. These effluvia reach the organ of smell- 

 ing, 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 parti- 

 cles 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 physical effect upon the organs of smelling. 



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 capa- 

 ble of expressing the effects of vision, according to the latter hypothesis. It 

 is evident, however, that as the first hypothesis requires us to admit that dis- 

 tant visible objects are continually ejecting matter from their surfaces to pro- 

 duce 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 pulsations or undu- 

 lations of distant visible objects and transmitting them to the eye. This hy- 

 pothetical fluid has been called the luminiferous ether. 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 experiments and observations only, adopted never- > 

 theless the nomenclature 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 undulating theory, on the other hand, was adopted by Huygens, and ( 

 after him by most continental philosophers. 



The researches in the phenomena of optics within the last hundred years have s 

 been marked by singular diligence and success. A vast variety of phenomena ) 

 previously unknown, have been accurately investigated, new laws have been \ 



