pHYSICAL any LITERARY. 87 
to a faint fea-green, by the mixture of his 
tranfmitted beams, which are then yel- 
lowith; with the ethereal blue: At greater 
diftances, this faint green gradually chan- 
ges into a reddifh brown; becaufe the 
fan ’s rays, by paffing thro’ more air, be- 
gin to incline to orange: And, on the 
oppofite fide of the hemifphere, the co-+ 
lour of thé horizontal fky inclines fenfibly 
to purple; becaufe his tranfmitted light 
which mixes with the azure, by pafling 
through a ftill greater length of air, be- 
’ comes reddifh; as we have faid above. 
To underftand diftinétly why the fun’s 
rays, by pafling through a greater and 
greater quantity of air, change by de- 
grees from white to yellow, thence to o« 
range, and laftly to red, we have only ta 
apply to the atmofphere, what Sir Ifaac 
fays (Book I. of his Optics, part 2. prop.. 
Io.) concerning the colour of tranfparent 
liquors in general. 
Is it not the fame coloured light of the 
rifing and fetting fun which tinctures the 
clouds, that, being thrown by the refrac- 
“tion of the atmofphere into the earth’s 
fhadow, 
