462 OCULAR SPECTRA. Sect. XL. 10. 3. 



circular area of red 01k placed on white paper, a lucid crefcent 

 or edge is feen to librate on one fide or other of the red circle : 

 for the exterior parts of the retina fometimes falling on the edge 

 of the central filk, and fometimes on the white paper, are lefs 

 fatigued with red light than the central part of the retina, which 

 is conftantly expofed to it ; and therefore, when they fall on the 

 edge of the red filk, they perceive it more vividly. Afterwards, 

 when the eye becomes fatigued, a green fpedtrum 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 filk appears paler, the edge on 

 which the unfatigued part of the retina occafionally falls will ap- 

 pear of a deeper red than the original filk, becaufe it is com- 

 pared with the pale internal part of it. M. de BufFon in mak- 

 ing this experiment obferved, that the red edge of the filk was 

 not only deeper coloured than the original filk ; but, on his re- 

 treating a little from it, it became oblong, and at length divided 

 into two, which muft have been owing to his obferving it either 

 before or behind the point of interferon of the two optic ax- 

 ifes. Thus, if a pen is held up before a diftant candle, when 

 we look intenfely at the pen two candles are feen behind it ; 

 when we look intenfely at the candle two pens are feen. If 

 the fight be unfleady at the time of beholding the fun, even 

 though one eye only be ufed, many images of the fun will ap- 

 pear, 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 djiappear fooner than the others > and hence the num- 

 ber and ihape of thefe fpeclra of the fun will continually vary, 

 as long as they exift. The caufe of fome being more vivid than 

 others, is the unfteadinefs of the eye of the beholder, fo that 

 fome parts of the retina have been longer expofed to the fun- 

 beams. That fome parts of a complicated fpettrum fade and 

 return before other parts of it, the following experiment evinces. 

 Draw three concentric circles ; the external one an inch and a 

 half in diameter, the middle one an inch, and the internal one 

 half an inch ; colour the external and internal areas blue, and 

 the remaining one yellow, as in Fig. 4. \ after having looked 

 about a minute on the centre of thefe circles, in a bright light, 

 the fpeclrum of the external area appears firft in the clofed eye, 

 then the middle area, and laftly the central one ; and then the 

 central one disappears, and the others in inverted order. If con- 

 centric circles of more colours are added, it produces the beau- 

 tiful ever changing fpe£!rum in Sccl. I. Exp. 2. 



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