66 ESSAYS anND OBSERVATIONS 
Ifaac’s method*, a beam of folar light, 
confifting of fuch colours and in fuch 
proportions as were feen in the lights of 
"falts and burning fpirits; and then to 
obferve in it the appearance of coloured: 
bodies? Further, are not the intervals of 
the fits, in rays of any one colour, the 
fame in the fame meduim, from whatever 
luminous body they are emitted? For, if 
thefe intervals were different, would there 
not be changes in the. colours of bodies, 
not to be accounted for by the eompoli- 
tions of the lights with which they are’ 
illuminated? | | 
QveErR. II. Donot all luminous bodies, 
the molt languid as well as moft bright, 
emit their lights of any one colour with one 
determinate velocity ; fince it is found by 
experience that they are all equally refrac- 
ted by the fame medium? And therefore, 
does not the different fplendor of lumi- 
nous bodies proceed wholly from the dif- 
ferent denfity of their light»at equal di- 
ftances? Andis not this confirmed by the 
equality’ 
* Newt: Opt, book r.-part 2. prop. If* 
