ocular SpeSira of Light and Colours • 
343 
XV. Variation of fpettra in refpeSl to brilliancy ; the vifibility of the 
circulation of the blood in the eye.. 
1. The meridian or evening light makes a difference in the 
colours of fome fpedra ; for as the fun defcends, the red rays, 
which are lefs refrangible by the convex atmofphere, abound 
in great quantity. Whence the fpedrum of the light parts of 
a window at this time, or early in the morning, is red ; and 
becomes blue either a little later or earlier; and white in the 
meridian day ; and is alfo variable from the colour of the clouds 
or iky which are oppofed to the window. 
2. All thefe experiments are liable to be confounded, if tkey 
are made too foon after each other, as the remaining fpedrurn 
will mix with the new ones-. This is a very troublefome cir- 
cum Ranee to painters, who are obliged to look long upon the fame 
colour; and in particular to thofe whofe eyes, from natural de- 
bility, cannot long continue the fame kind of exertion. For 
the fame reafon, in making thefe experiments, the refult be- 
comes much varied if the eyes-, after viewing any objed, are 
removed on other objeds for but an inftant of time, before we 
clofe them to view the fpedrurn for the light from the objed 9 , 
of which we had only a tranfient view, in the very time of 
doling our eyes ads as a fifimulus on the fatigued retina ; and 
for a time prevents the defired fpedrurn from appearing, or 
mixes its own fpedrurn with it. Whence, after the eyelids^ 
are clofed, either a dark field, or fome unexpeded colours, are 
beheld for a few feconds, before the defired fpedrurn becomes 
diftindly vifible. 
^ The length of time taken up in viewing an objed, of 
which we are to obferve the fpedrum, makes a great difference 
in 
