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LECTURE XXXIX. 



ON THE NATURE OF LIGHT AND COLOURS. 



X HE nature of light is a subject of no material importance to the concerns of 

 life or to the practice of the arts, but it is in many other respects extremely in- 

 teresting, especially as it tends to assist our views both of the nature of our sen- 

 sations, and of the constitution of the universe at large. The examination of 

 the production of colours, in a variety of circumstances, is intimately con- 

 nected with the theory of their essential properties, and their causes ; and we 

 shall find that many of tliese phenomena will afford us considerable assistance 

 in forming our opinion respecting the nature and origin of light in general. 



It is allowed on all sides, that' light either consists in the emission 

 of very minute particles from luminous substances, which are actually pro- 

 jected, and continue to move, with the velocity commonly attribut- 

 ed to light, or in the excitation of an undulatory motion, analogous to 

 that which constitutes sound, in a highly light and elastic medium pervading 

 the universe; but the judgments of philosophers of all ages have been 

 much divided with respect to the preference of one or the other of these opi- 

 nions. There are also some circumstances which induce those, who entertain 

 the first hypothesis, either to believe, with Newton, that the emanation of 

 tlie particles of light is always attended by the undulations of an etherial 

 medium, accompanying it in its passage, or to suppose, with Boscovich, 

 that the minute particles of light themselves receive, at the time of their 

 emission, certain rotatory and vibratory motions, which they retain as long 

 as their projectile motion continues. These additional suppositions, how- 

 ever necessary they may have been thought for explaining some particular 

 phenomena, have never been very generally understood or admitted, although 

 no attempt has been made to accommodate the theory in any other manner to 

 tiiose phenomena. 



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