214 Sir D. Brewster on the Connexion bePweqn 



The reflected tints of course vary with the obliquity of the 

 incident light ; and at great incidences the transmitted ones, 

 however splendid and varied, all become pale yellow. When 

 these combinations of glass films are immersed in a balsam or 

 an oil, their colours, whether transmitted or reflected, all dis- 

 appear, excepting a pale yellow light like that which is trans- 

 mitted at great incidences. These facts prove, beyond a 

 doubt, that the transmitted colours, though wholly unlike 

 to those of thin plates, are yet produced by the same cause, 

 and are residuary, and generally complementary to the hue 

 of the reflected tints. 



The analysis of these colours by the prism affords a series 

 of most beautiful and instructive phaenomena, and it is only 

 by coloured drawings that any adequate idea of them can be 

 conveyed. All the phenomena of coloured media, with bands 

 of various breadths and various intensities of illumination, are 

 exhibited in great perfection, so as to identify completely in 

 this feature the two classes of facts. But what is still more 

 striking, the colours of the bands are changed, and we thus 

 find that the characteristic phaenomenon of absorption is pro- 

 duced by the action of thin plates. To such a degree indeed 

 is the change of tint carried, that I have insulated a white band 

 in the orange part of the spectrum. 9 



Notwithstanding this identification of absorption and pe- 

 riodical action in their primary features, there are two points 

 of difference which separate widely the two classes of phaeno- 

 mena : the first of these is, that the bands and tints of ab- 

 sorbing media are not changed by obliquity; and the second, 

 that the reflected tints are not visible in such media. Sir 

 Isaac Newton endeavoured to remove the first of these diffi- 

 culties by supposing that the particles of bodies on which 

 their colours depended have an enormous refractive power ; 

 and M. Biot * has endeavoured to meet it more effectually by 

 introducing two new suppositions ; viz. that the particles are 

 capable of transmitting light only through their centre of gra- 

 vity, and that the lateral transmissions may be prevented or 

 turned aside by the inflecting forces which act at a distance 

 on the luminous molecules which approach them. 



These explanations of the uniformity of the tints at all in- 

 cidences have been rendered necessary, not perhaps by the 

 real difficulties of the case, but in consequence of Sir Isaac 

 Newton and his followers taking it for granted that the co- 

 lours of natural bodies were pure tints of a particular order. 

 Hence it becomes a necessary assumption in the theory that 



* Traite de Physique, torn. iv. p. 126. 



