Inquiries into Coloured Light, 47 



fublinied and purer vapours than our earth does; and fo ex- 

 hibit purer and more homogeneous coloured lijrhts thju the 

 terreftrial inflamed vapours do, although in every other re- 

 fpect of a fimilar nature? May not this explain' the reafon 

 why fomc of the fixed (lars, alio funs, appear of a screen or 

 red colour? for, if their bodies confift of a difTerciu mine- 

 ralogy, and the vapours of iheir atmofpheres be thus difpofed 

 to throw out diflerent, as for inllance green and redlio;hts, in 

 a greater proportion than the other colours, they will thus, 

 or courfc, have this or that appearance of being green or red; 

 which is a known phaenomenon. Nay, even the white lucid 

 light of the fun, apart all confideration of circumftances which 

 may aftecl jt as it comes through our atmolphere, gives out at 

 times a fomewhat diHerently futfufed white, according as this 

 or that conftituent of its compound light may be in a gteater 

 or Icfs proportion. 



Alay not the moon, producing an atmofphere vifible where 

 no light of the fun can reach, and therefore not a reflected 

 light, polfefs, as a plane,t, an atmofphere very different from 

 what thole funs produce, perhaps of a phofphorefcent nature, 

 which exifts with little heat ? And may not the vifible 

 atmofphere of Venus, confilting of enlightened vapours, be of 

 the fame fort ? I will ftrcngthen the proportion of tiiis ((u^ere 

 from the opinion of Dr. lierfchel on this planet : " It cannot 

 Ihine (he fay<) by a borrowed light, fo that this faint illu- 

 mination mull denote fome phofphoric quality in the atmo- 

 lj)here of V^enus." 



Thus far I proceed in my refearches on the ground of the 

 folar lioht being a compound of various rays, fuppofed to be 

 originally difpofed to exhibit feven or three primary colours; 

 which colours arc alfo fuppofed to be homogeneous.' A doubt, 

 however, has always occurred to me, firll, whether this opi- 

 nion of the homogeneity of the feven primary colours be 

 founded in fatA : and, in the courfc of exainin'ing this firlt 

 doubt, a. further doubt arifes, whether there snxjLVt'n or only 

 Mr^:^ principal colours, as Sir Ifaac Newton cxprefles it ; or 

 whether there be more than two, and perhaps nut more than 

 one. Stating then what follows as a doubt followed up by 

 inyelligation, it appears to me, that the fpace which each 

 prifmatic primary colour (as it is called) lakes in the prifmatic 

 fpcclrum, has not one only angle of refradion within its ex- 

 lent, but a fucceffivc feries of angles of refradlion, and a fuc- 

 ceflive feries * of innumerable circular or orbicular images of 

 the fun (as Sir Ifaac Newtou ftatcs the fad), fo clofdy later- 



OJ•tJL^, experiment 5, p. 34, HJc Eg. 15. j^Iitc j. of book i. part i. 



feeling 



