ocular Speblra of Light and Colours* 345 
to produce this curious phenomenon, if one hand be brought 
nearer the clofed eyelids than the other, the circulation in that 
eye will for a time difappear. For the eafier viewing the circu- 
lation, it is fometimes neceffary to rub the eyes with a certain 
degree of force after they are clofed, and to hold the breath 
rather longer than is agreeable, which, by accumulating more 
blood in the eye, facilitates the experiment ; but in general it 
may be feen diftindlly after having examined other fpedtra with 
your back to the light, till the eyes become weary ; then, 
having covered your clofed eyelids for half a minute, till the 
fpedtrum is faded away which you were examining, turn your 
face to the light, and removing your hands from the eyelids, 
by and by again (hade them a little, and the circulation be- 
comes curioufly diftindt. The ftreams of blood are however 
generally feen to unite, which fhews it to be the venous circu- 
lation, owing, I fuppofe, to the greater opacity of the colour 
of the blood in thefe veffels ; for this venous circulation is alfo 
much moreeafily feen by the microfcope in the tail of a tadpole. 
v. Variation of fpedtra in refpeft to diflinZinefs and fze ; with a 
new way of magnifying objects. 
1. It was before obferved, that when the two colours viewed 
together were oppofite to each other, as yellow and blue, red 
and green, &c. according to the table of refiedtions and tranf- 
miffions of light in Sir Isaac Newton’s Optics, B. II. fig. 
3. the fpedtra of thofe colours were of all others the mod bril- 
liant, and beft defined ; becaufe they were combined of the 
reverfe fpedtrum of one colour, and of the diredt fpedtrum of 
the other. Hence, in books printed with Tmall types, orfin 
Vol. LXXYL Z z ■ , the 
