386 On the Number of the Primitive Colonfic Rays in Solar Light. 



chap. I. § 3. Befides that this is a mere hypothefis, unfupported by any fa£l, as Fourcroy 

 obferves, we remark, that it is in itfeK inadequate j ift, becaufe in the folar fpeftrum, the 

 red and indigo are riot neighbouring colours, but are almoft at the greateft poffible diftance 

 from each other. 2dly, According to this hypothefis, indigo is compofed of blue and vio- 

 let J but violet is compofed of red and indigo ; indigo therefore is compofed of red, blue 

 and indigo, that is, indigo itfelf is one of its own eflential ingredients, which is abfurd. 



The experiments of the prifm feeni to eftablilh, in a very clear liianner, the exiftence of 

 feven original and uncompounded colours; and though green, for inftance, may be com- 

 pounded of blue and yellow, yet it does not direftly follow from thence, that it always is 

 fo a£lually compounded. Accordingly Newton tells us, that green may be exhibited in 

 two different ways, either by primitive, green-making rays, which are fimple and not re- 

 folvable by any refleftion or refra£lIon into different rays ; or by a compofition of blue and 

 yellow ray», which are differently refrangible, and which therefore after their union, may 

 again be feparated by refraftion, and exhibit their proper colours of blue and yellow. 



On this dodrine of the two-fold generation of green, we may in the flrft place remark, 

 that the ancient, received axiom " Deus nit agitfuftra" ought not to be too haftily aban- 

 doned, as it mull appear to be, if this doftrine be maintained : for if green may be pro- 

 duced by blue and yellow, then blue and yellow being already exiftent, green is a confe- 

 quence ; and therefore peculiar rays formed for the produ£lion of green are fuperfluous. 

 Though I acknowledge, that this maxim is not fo cogent or felf-evident, as to preclude all 

 objeftion, yet fince the general obfervation of nature feems to (hew, 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 lead fo far as this, that we Ihould be careful in fhewing, 

 that we are led to thefe different caufcs of the fame effedl, by a legitimate and cautious 

 analyfis. 



In defence of the doftrine of three primitive colours only, F. Caftelli contents himfelf 

 with faying, that the colours of the prifm are immaterial, accidental, artificial, and there- 

 fore unworthy the regard of a phllofopher ; whereas the colours of painters are fubftantial, 

 natural, palpable. From them, of confequence, the theory of chromatics (hould be de- 

 duced J but they 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 j and therefore that 

 our theory of colours muft be derived from an enquiry into the conftitution 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 phenomena is unqueftionably falfe. 



I fhall therefore proceed to enquire fcrupuloufly into the compofition of the folar fpec- 

 trum, from which, without doubt, the true doftrine of the origin of colouifs is to be 

 derived^ 



If 



