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rated by refraaion, and exhibit their proper colours of blue and 

 yellow. 



Os this dodtrine of the two-fold generation of green, we may 

 in the firft place remark, that the antient, received axiom " Deus 

 " nil agit fuflra" ought not to be too haftily abandoned, as it 

 muft appear to be, if this dodrine be maintained : for if green 

 may be produced by blue and yellow, then blue and yellow 

 being already exiftent, green is a confequence ; and therefore 

 peculiar rays formed for the produdion of green are fuperfluous. 

 Though I acknowledge, that this maxim is not fo cogent or felf- 

 evident, as to preclude all objedion, yet fince the general obfer- 

 vation of nature feems to fhew, that this wafte of power or 

 multiplicity of means is not adopted by the Supreme Artift, 

 it certainly feems juftly entitled to our attention, at leaft fo far as 

 this, that we fliould be careful in fhewing, that we are led to 

 thefe different caufes of the fame effed, by a legitimate and 

 cautious analyfis. 



In defence of the dodrine of three primitive colours only, 

 F. Caftelli contents himfelf with faying, that the colours of the 

 prifm are immaterial, accidental, artificial, and therefore un- 

 worthy the regard of a philofopher ; whereas the colours of 

 painters are fubftantial, natural, palpable. From them, of con- 

 fequence, the theory of chromatics Ihould be deduced ; bvit they 



Vol. VII. Q^ teil 



