f)g METHOD OF EXAMINING DISFERStVE POWERS.' 



I have endeavoured to reduce the feveral fubftances thus exa- 

 mined to one table; but, as the limits of colour are in few in- 

 ftances iufficiently well defined for accurate menfuration, I have 

 not attempted to add any numerical eftimate of their powers, 

 but have merely afcerlained the order in which thay fucceed 

 each other ; and, in the following table, have arranged them 

 according to the excefs of their effect on violet above red light, 

 at a given angle of deviation. 



E III. 



Numerous By cpmparifon of this table with the order of refractive 



achromatic com- powers, as contained in the' firft table, it will be feen how little 



correfpondence there is between them ; and, accordingly, how 



numerous are the combinations by means of which a pencil of 



rays that partes through two media, may be made to deviate 



without difperfion of its colours. 



White light does I cannot conclude thefe obfervations on difperfion, without 



not appear to be remarking that the colours into which a beam of white light is 



P^im* inio fcw» ^ e P ara ble by refraction, appear to me to be neither 7, as they 



nor into three ufually are feen in the rainbow, nor reducible by any means 



(that I can find) to 3, as fome perfons have conceived ; but 



that, by employing a very narrow pencil of light, 4 primary 



divifions of the prifmatic fpeclrum may be feen, with a degree 



of diftinclnefs that, I believe, has not been defcribed nor ob- 



ferved before. 



4 If 



colours. 



