38 A TREATISE 





is next to the circumference j and the blew, on that fide, which 

 is next the a\ii : but if it be placed on one fide of the axis it\ the 

 fccond cone, then the blew will appears on that fide thepJciure 

 which is next the circumference; and thered,on that fide,which 

 is next the center of the bafis of the COJKT. 



* There remaincth yet one difficulty of moment to be detcrtni- 



b! diffiCU fol P ed liet * : vvfl ' c k ** w hy* wnn through a glafle,tvYO colours (namely, 



touching! he 6 ^lew am ^ reci ) are ca(t from a canclle "P n a P a Pr or wall , "if 



prifmc, you put your eye in the place of one of the colours that fhineth 



upon the wall., and fo that colour commeth to fliine upon your 



eye, in fuch fort that another man who looketh upon it, will fee 



that coJour plainly upon your eye , nevertheJeffe, you fhali fee 



the ether colour in the glaflfe ? As for example, if on your eye 



there fhmeth a red.you (hall fee a blew in the glafleiand ifa blew 



ftiineth upon your eye,you mall fee a red. 



The rcafon hereof is , that the colours which appeare in the 

 glafle.,are of the raturc of thofe luminous colours which we firrt 

 explicated,that arife from looking upon white and blacke bor- 

 dering together : for a candle ftanding in the ayre, is as it were 

 a white fituated betweene two blackes : the circumftant dusky 

 ayre , having the nature of a blacke : fo then , that fide of the 

 candle which is feen through the thicker part of the glafle , ap- 

 peared red ; and that which is feen through the thinner,appea- 

 rethblew: in the fame manner as when we lonke through the 

 * glaiTe ; whereas^the colours fhine contrarywile upou a paper or 



reflefting objed 5 as we have already declared, together wuh the 

 rcafom of both thefe appearances ; each fitted to its proper cafe, 

 of looking through the glafTe upon the luminous object fur- 

 rounded with darknefle, in the one ; and of ebierving theeffecl 

 wrought by the fame luminous object in fome medium or upon 

 fome refle&ent fuperficies,in the other. 



And to coafirme this, if a white paper be let ftanding hollow 

 before the glafTe ( like halfe a hollow pillar, whoie flat ftandcth 

 edgcwaies towards the glafle, fo as both the cd^es may be feen 

 through it) the further edge will feem bJew, and thencerer will 

 be red; and the like will happen , if the paper be held in the free 

 ayreparallell to the lower fuperficics of the glafle, without any 

 bdackecarpet to limit both ends of it (which ferveth to make the 

 colour* the fmartcr) 16 that in boch cafc5,thc ayrc ferveth manj- 



feftjy 



