A TREATISE 



fo as to multiply the light which goetfcthrowgh the prifme 

 to the paper , you deftroy much of the colour by converting it 

 into light. But on the other fide, if you thicken the ayre, and 

 make it dusky with fmoake, or wich duft ; you will plainely fee, 

 that where the light commeth through a convexeelaflc ( per- 

 pendicularly oppoied to the illummant) there wiliappeare co- 

 lours on the edges of the cones that the light maketh : and per- 

 adventure the whole cones would appeare coloured if the dark- 

 ning were conveniently made : for if anopacous body be let 

 within either of the cones , its h'des \viir~appeare coloured, 

 though the ay re be but moderately thickned : which Ihevveth 

 that the addition of a little darknefle , would make that which 

 otherwiie appeareth pure light, be throughly dyed into colours. 

 And thus you have the true and adequate cau/eof the appea- 

 rance of fach colours. 



^ Now.,to understand what colours, and upon which fides, will 



Upon what appeare ; we may coniider , that when light pafleth through a 

 fide etery co glafle,or other diaphanous body, fo much of it as fhineth in the 

 lout appetreth a y re)Or U p On feme reflecting body bigger then it fclfej, after its 

 pureTght paflagethrou^h the glaflfe , muft of necerHty have darknefle on 

 ng through both lidcs of it ; and fo be comprised and limited by two dark- 

 c. neflfes : butiflbmeopacousbody,that isJeflcthen'theligh^ be 

 put in the way of the light a then it may happen contrariwi/e, that 

 there be darknefle ( or the madow of that opacous body ) be- 

 - tween two lights. 



Againe , we muft con(ider 3 that when light falleth fb upon a 

 prifme as to make colours, the two outward rayes which pro- 

 ceed from the light to the two fides of the faperficies at which 

 the light entcreth , are fo refracted that at their commiflg out 

 againe through the other fuperficies , that ray which made the 

 lefle angle with the outward fuperficiesof the glaflfe, going in, 

 maketh the greater angle with the outfide of the other fuperfi- 

 cies,commingout : and contrariwife , that ray which made the 

 greater angle, going in, maketh the lefler,at its comming out : 

 and the two internall angles, made by thofe two rayes , and the 

 outfide of the fuperficies they ifiue at,arc greater then two right 

 ingles : and fo we fee that the light dilatcth it felfe at its com- 

 mingout. 



Now, bcciuic rayes chat i^fuc through a fpcrficics,ihc neerer 

 \ they 



