PHYSICAL ann LITERARY. sy. 
‘change the colour of any fimple ray. But 
it appears, from what we have now faid, 
that none of hisexperiments prove the im- 
mutability of fimple rays by the fecond 
effects of refraction. Pp 
47. As it is of great confequence in 
philofophy to diftinguifh between facts 
and hypothefes, however plaufible, it 
ought to be obferved, that the various re- 
frangibility, reflexibility, and inflexibility 
of the feveral colours, and their alternate 
difpofitions at equal intervals to be reflect- 
ed and tranfmitted, which are the whole 
ground-work of the Newtonian fyftem, 
are to be confidered as certain facts de- 
duced from experiment: But, whether the 
velocities of the different rays are exactly 
equal, or different in the manner now de= 
feribed, is no more than probable con- 
jecture ; and though this point fhould be 
decided by a method propofed afterwards, 
it would {till continue uncertain, whether 
the fits of reflexion and tranf{miffion are 
eccafioned by an alternate acceleration 
and retardation of the motion of light, 
OF. 
