SECT. XL. 19. OCULAR SPECTRA. 633 



it more vividly. Afterwards, when the eye becomes 

 fatigued, a green fpectrum in the form of a crefcent 

 is feen to librate on one fide or other of the central 

 circle, as by the unfteadinefs of the eye a part of 

 the fatigued retina falls on the white paper; and as 

 by the increafing fatigue of the eye the central part 

 of the iilk appears paler, the edge on which the 

 unfatigued part of the retina occasionally falls will 

 appear of a deeper red than the original filk, be- 

 caufe it is compared with the pale internal part of 

 it. M. deBuffon in making this experiment obferv- 

 ed, that the red edge of the filk was not only deeper 

 coloured than the original filk; but, on his retreat 

 ing a little from it, it became oblong, and at length 

 divided into two, which mud have been owing to 

 his obferving it either before or behind the point of 

 intersection of the two optic axifes. Thus, if a pen 

 is held up before a diftant candle, when we look in- 

 tenfely at the pen, two candles are feen behind it ; 

 when we look intenfely at the candle, two pens are 

 feen. If the fight be unfteady at the time of be- 

 holding the fun, even though one eye only be ufed, 

 many images of the fun will appear, or luminous 

 lines, when the eye is clofed. And as fome parts 

 of thefe will be more vivid than others, and fome 

 parts of them will be produced nearer the centre of 

 the eye than others, thefe will difappear fooner than 

 the others ; and hence the number and fhane of 

 ihefe fpectra of the fun will continually vary, as 

 long as they exift. The caufe of fome being more 

 .vivid than others, is the unfteaciinefs of the eye of 

 the beholder, fo that fome parts of the retina have 

 been longer expofed to the funbeams. That fome 

 parts of a complicated fptctrum fade and return be- 

 lore other parrs of it, the following experiment 

 evinces. Diaw three concentric circles ; the exter- 

 nal one an inch and a half in diameter, the middle 

 one an inch, and the internal one half an inch j co- 

 T t 2 lour 



