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tell us, that there are but three parent colours, which give birth to 

 all others. 



In reply to this we need only obferve, that Sir I. Newton has 

 proved, that the colours of natural bodies depend on the colorific 

 qualities of the rays of light ; and therefore that our theory of 

 colours muft be derived from an enquiry into the conftltution of 

 folar light, for according to that conftitution the colours of bodies 

 will vary : and he farther fhews, that if folar light confifted of 

 but one fort of rays, all bodies in the world would be of the fame 

 colour. However true therefore F. Caftelli's theory may be, the 

 manner in which he deduces it from phsenomena is unqueftion- 

 ably falfe. 



I Ihall therefore proceed to enquire fci-upuloufly into the com- 

 pofition of the folar fpedrum, from which, without doubt, the 

 true dodtrine of the origin of colours is to be derived. 



If the folar light confifted of feven primitive, homogeneal co- 

 loured rays, and that thefe homogeneal rays were equally re- 

 frangible, the fpedrvim would confift of feven circles of different 

 colours, fince the homogeneal rays of each colour would paint 

 a circular image of the fun. But it is manifeft, that feven circles 

 could not compofe an oblong fpecflrum, with redilineal fides. 

 Therefore the rays of the fame denomination of colour muft 

 be differently refrangible. Which is alfo made flill farther 



evident 



