SicT.XL. io. OCULAR SPECTRA. 63$ 



colour ; and in particular to thofe whofe eyes, from 

 natural debility, cannot long continue the fame kind 

 of exertion. For the fame reafon, in making thefe 

 experiments, the refult becomes much varied if the 

 eyes, after viewing any object, are removed on other 

 objects for but an inftant of time, before we clofe 

 them to view the fpe&rum ; for the light from the 

 object, of which we had only a transient view, in 

 the very time of clofing our eyes acts as a flimulus 

 on the fatigued retina; and for a time prevents the 

 defircd fpeclrum from appearing, or mixes its own 

 fpe&rum with it. Whence, after the eyelids are 

 clofed, either a dark field, or fome unexpected co- 

 lours, are beheld for a few feconds, before the de- 

 fired fpectrum becomes diftindlly vifible. 



3. The length of time taken up in viewing an 

 object, of which we are to obferve the fpe&rum, 

 makes a great difference in the appearance of the 

 fpeclrum, not only in its vivacity, but in its colour ; 

 as the direct fpe&rum of the central object, or of 

 the circumjacent ones, and alfo the reverfe fpectra 

 of both, with their various combinations, as well as 

 the time of their duration in the eye, and of their 

 remimons or alternations, depend upon the degree 

 of iatigue the retina is fubjected to. The Chevalier 

 d'Arcy conftructed a machine by which a coal of 

 fire was whirled round in the dark, and found, that 

 when a luminous body made a revolution in eight 

 thirds of time, it prefented to the eye a complete 

 circle of fire ; from whence he concludes, that the 

 impreflion continues on the organ about the feventh 

 part of a fecond. (Mem. de 1'Acad. des Sc. 1765.) 

 This, however, is only to be conlidered as the fhort- 

 eft time of the duration of thefe direct fpectra ; 

 fince in the fatigued eye both the direct and reverfe 

 fpectra, with their intermirlions, appear to take up 

 many feconds of time, and feem very variable ia 

 proportion to the circumftances of fatigue or energy. 



4 . it 



