464 OCULAR SPECTRA. Sect. XL. ic. 4. 



body made revolution in eight thirds of time, is prefented to the 

 eye a complete circle of fire ; from whence he concludes, that 

 the impreilion continues on the organ about the feventh part of 

 a fecond. (Mem. de l'Acad. des Sc 1765.) This, however, 

 is only to be confidcred as the ihorteft time of the duration of 

 thefe direct fpectra ; fince in the fatigued eye both the direct 

 and reverie fpectra, with their intermiflions, appear to take up 

 many .feconds of time, and feem very variable in proportion to 

 the circumftances of fatigue or energy. 



4. It fometimes happens, if the eyeballs have been rubbed 

 hard with the fingers, that lucid fparks are ken in quick mo- 

 tion amidft the fpectrum we are attending to. This is fimilar 

 to the flames of fire from a (Iroke on the eye in fighting, and is 

 refembled by the warmth and glow, which appears upon the fkin 

 after friction, and is probably owing to an acceleration of the ar- 

 terial blood into the veiTels emptied by the previous prefTure. By 

 being accuftomed to obferve fuch fmall fenfations in the eye, it 

 is eafy to fee the circulation of the blood in this organ. I have 

 attended to this frequently, when I have obferved my eyes more 

 than commonly fenfible toother fpectra. 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 neceflary to produce this 

 curious phenomenon, if one hand be brought nearer the clo- 

 fed eyelids than the other, the circulation in that eye will for a 

 time difappear. For the eafier viewing the circulation, 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 diftinctly after having examined other fpectra with your 

 back to the light till the eyes become weary •, then having cover- 

 ed your clofed eyelids for half a minute, till the fpectrum is fa- 

 ded 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 becomes curioufly diftinct. 

 The ftreams of blood are however generally (een to unite, 

 which (hews it to be the venous circulation, owing, I fuppofe, 

 to the greater opacity of the colour of the blood in thefe veiTels ; 

 for this venous circulation is alfo much more eafily feen by the 

 microfcope in the tail of a tadpole. 



5. Variation offpefira in refpecl to dijtinrfnefs andjize ; with a new 



way of magnifying objects. 



I. It was before obferved, that when the two colours viewed 



together 



