359 



XECTURE XXXIX, 



ON THE NATURE OF LIGHT AND COLOURS. 



THE nature of light is a subject of no material importance to the con- 

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

 extremely interesting, especially as it tends to assist our views both of the 

 natflre of our sensations, and of the constitution of the universe at large. 

 The examination of the production of colours, in a variety of circum- 

 stances, it f:?timately connected with the theory of their essential properties, 

 and their causes ; and we shall find that many of these phenomena will 

 afford us considerable assistance in forming our opinon 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 projected, 

 and continue to move with the velocity commonly attributed to light, or 

 in the excitation of an undulatory motion, analogous to that which con- 

 stitutes 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 opinions. There are 

 also some circumstances which induce those, who entertain the first hypo- 

 thesis, either to believe, with Newton,* that the emanation of the par- 

 ticles of light is always attended by the undulations of an etherial medium, 

 accompanying it in its passage, or to suppose, with Boscovich,t 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, however neces- 

 sary they may have been thought for explaining some particular pheno- 

 mena, have never been very generally understood or admitted, although no 

 attempt has been made to accommodate the theory in any other manner to 

 those phenomena. 



We shall proceed to examine in detail the manner in which the two 

 principal hypotheses respecting light may be applied to its various proper- 

 ties and affections ; and in the first place to the simple propagation of light 

 in right lines through a vacuum, or a very rare homogeneous medium. In 

 this circumstance there is nothing inconsistent with either hypothesis ; but 

 it undergoes some modifications, which require to be noticed, when a por- 

 tion of light is admitted through an aperture, and spreads itself in a slight 

 degree in every direction. In this case it is maintained by Newton that 

 the margin of the aperture possesses an attractive force, which is jcapable 

 of inflecting the rays : but there is some improbability in supposing that 

 bodies of different forms and of various refractive powers should possess 

 an equal force of inflection, as they appear to do in the production of these 



* * Ph. Tr. vii. 5087. 



f Dissertatio de Lumine, Part II. 1748 ; and Theoria Philosopbia Naturalis, 4to, 

 'Venice, 1763, p. 230. 



