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LIGHT. 



8. That the forces which produce the reflection arid refraction of light are, 

 nevertheless, absolutely insensible at all measurable or appreciable distances 

 from the molecules which exert them. 



9 That every luminous molecule, during the whole of its progress through 

 space, is continually passing through certain periodically recurring states, called 

 by Newton fits of easy reflection and easy transmission, in virtue of which 

 they are more disposed, when in the former states or phases of their periods, 

 to obey the influence of the repulsive or reflective forces of the molecules of a 

 medium ; and when in the latter, of the attractive. 



Such are the principles necessary to be admitted in the corpuscular theory. 

 Herschel states those of the undulatory theory as follows : 



1. That an excessively rare, subtle, and elastic medium, or ether, fills all 

 space, and pervades all material bodies, occupying the intervals between their 

 molecules ; and either by passing freely among them, or by its extreme rarity, 

 offering no resistance to the motion of the earth, the planets, or comets, in their 

 orbits, appreciable by the most delicate astronomical observations ; and having 

 inertia, but not gravity. 



2. That the molecules of the ether are susceptible of being set in motion by 

 the agitation of the particles of ponderable matter ; that when any one is thus 

 set in motion, it communicates a similar motion to those adjacent to it : and 

 that the motion is propagated farther and farther in all directions, according to 

 the same mechanical laws which regulate the propagation of undulations in 

 other elastic media, as air, water, or solids, according to their respective con- 

 stitutions. 



3. That in the interior of refracting media the ether exists in a state of less 

 elasticity, compared with its density, than in vacuo (that is, space empty of all 

 other matter) ; and that the more refractive the medium, the greater, relatively 

 speaking, is the elasticity of the ether in its interior. 



4. That vibrations jominanicated to the ether in free space are propagated 

 through refractive media by means of the ether in their interior, but with a ve- 

 locity corresponding to its inferior degree of elasticity. 



5. That when regular vibratory motions of a proper kind are propagated 

 through the ether, and, passing through our eyes, reach and agitate the nerves 

 of our retina, they produce in us the sensation of light, in a manner bearing a 

 more or less close analogy to that in which the vibrations of the air affect our 

 auditory nerves with that of sound. 



6. That as, in the doctrine of sound, the frequency of the aerial pulscc, or 

 the number of excursions to and fro from the point of rest made by each mole- 

 cule of the air, determines the pitch or note ; so, in the theory of light, the 

 frequency of the pulses, or number of impulses made on our nerves in a given 

 dine by the ethereal molecules next in contact with them, determines the color 

 of the light ; and that as the absolute extent of the motion to and fro of the par- 

 ades of air, determines the loudness of the sound, so the amplitude or extent of 

 he excursions of the ethereal molecules from their points of rest determines 

 'he brightness or intensity of the light. 



Whichever theory we adopt to explain the phenomena of light, we are led to 

 conclusions that strike the mind with astonishment. According to the corpus- 

 cular theory, the molecules of light are supposed to be endowed with attractive 

 and repulsive forces, to have poles to balance themselves about their centres 

 of gravity, and to possess other physical properties which we can only ascribe 

 to ponderable matter. In speaking of these properties, it is difficult to divest 

 oneself of the idea of sensible magnitude, or by any strain of the imagination 

 to conceive that particles to which they belong can be so amazingly small as 

 those of light demonstrably are. If a molecule of light weighed a single grain, 



