SECT. XL. 4. 2. OCULAR SPECTRA. 451 



of pafteboard ; on looking through the tube at a diftant prof- 

 per, fome disjointed parts of it will be feen through the nar- 

 row intervals between the fails ; but as the fly begins to revolve 

 thefe intervals appear larger ; and when it revolves quicker, the 

 whole profpeft is feen quite asdiftincl as if nothing intervened, 

 though lefs luminous. 



2. Look through a dark tube, about half a yard long, at the 

 area of a yellow circle of a half an inch diameter, lying upon a 

 blue area of double that diameter, for half a minute ; and on 

 clofing your eyes the colours of the fpeclrum will appear fimilar 

 to the two areas, as in fig. 3. ; but if the eye is kept too long 

 upon them, the colours of the fpeclrum will be the reverfe of 

 thofe upon the paper, that is, the internal circle will become 

 blue, and the external area yellow ; hence fome attention is re- 

 quired in making this experiment. 



3. Place the bright flame of a fpermaceti candle before a black 

 object in the night ; look ileadily at it for a (hort time, till it is 

 obferved to become fomewhat paler , and on clofing the eyes, 

 and covering them carefully, but not ib as to comprefs them, the 

 image of the blazing candle will continue diftinetly to be vitlble, 



Look ileadily, for a fhort time, at a window in a dark day, as 

 inExp. 2. Seel. III. and then clofing your eyes, and covering 

 them with your hands, an exact delineation of the window re- 

 mains for fome time vifible in the eye. This experiment re- 

 quires a little practice to make it fucceed well ; fince, if the 

 eyes are fatigued by looking too long on the window, or the day 

 be too bright, the luminous parts of the window will appear 

 dark in the fpectrum, and the dark parts of the frame- work will 

 appear luminous, as in Exp. 2. Seel. III. And it is even diffi- 

 cult for many, who firft try this experiment, to perceive the 

 fpeelrum at all ; for any hurry of mind, or even too great atten- 

 tion to the fpeclrum itfelf, will difappoint them, till they have 

 had a little experience in attending to fuch fmall fenfations. 



The fpeetra defcribed in this feelion, termed direct ocular 

 fpeetra, are produced without much fatigue of the eye ; the ir- 

 ritation of the luminous objeet being foon withdrawn, or its 

 quantity of light being not fo great as to produce any degree of 

 uneafmefs in the organ of vifion ; which diftinguifhes them 

 from the next clafs of ocular fpeclra, which are the confequence 

 of fatigue. Thefe diredl fpeclra are belt obferved in fuch cir- 

 cumftances that no light, but what comes from the objeet, can 

 fall upon the eye ; as in looking through a tube, of half a yard 

 long, and an inch wide, at a yellow paper on the fide of a room, 

 the direct fpectrum was eafily produced on clofing the eye with- 

 out taking it from the tube 5 but if the lateral light is admitted 



through 



