636 OCULAR SPECTRA. SECT. XL. 10. 



4. It fometimes happens, if the eyeballs have been 

 rubbed hard with the fingers, that lucid fparks are 

 feen in quick motion amidft the fpe&rum we are 

 attending to. This is fimilar to the flafhes of fire 

 from a ftroke on the eye in fighting, and is refem- 

 bled by the warmth and glow, which appears upon 

 the ikin after friction, and is probably owing to an 

 acceleration of the arterial blood into the veflels 

 emptied by the previous preflure. By being accuf- 

 tomed to obferve fuch fmall fenfations in the eye, it 

 is eafy to fee the circulation of the blood in this or- 

 gan. I have attended to this frequently, when I 

 have obferved my eyes more than commonly fenfi- 

 ble to other fpedra. The circulation may be feen 

 either in both eyes at a time, or only in one of 

 them ; for as a certain quantity of light is neceflkry 

 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 circulation, it is fome- 

 times neceflkry to rub the eyes with a certain de- 

 gree of force after they are clofed, and to hold the 

 breaih rather longer than is agreeable, which, by 

 accumulating more blood in the eye, facilitates the 

 experiment ; but in general it may be feen diftincl- 

 ly after having examined other fpe&ra with your 

 back to the light, till the eyes become weary ; 

 then having covered your clofed eyelids for half a 

 ininuie, till ihe fpe&rum is faded away which you 

 were examining, turn your'face to the light, and 

 removing your hands from the eyelids, by and by 

 again fhade theih a little, and the circulation be- 

 comes curioufly <iiiHn&. The dreams of blood are 

 however generally feen to uniie, which fhews it to 

 be the venous circulation, owing, I fuppofe, to the 

 greater opacity of the colour of the blood in thefe 

 . vcfllis ; for ihis venous circulation is alfo much 

 mo'.-c eafily feen by the microfcope in the tail of a 

 tadpole- 



