626 OCULAR SPECTRA. SECT. XL. 10. 



fpectra was owing to their being excited both by the 

 ftimuius of the interior and exterior objects ; fo that 

 the interior yellow fpectrurn was both the reverfe 

 fpectrum of the blue paper, and the direct one of 

 the yellow paper ; and the exterior blue fpectrum 

 was both the reverfe fpectrum of the yellow paper, 

 and the direct one of the blue paper. 



e. When the internal area was only a fquare half- 

 inch of red paper, laid on a fquare foot of dark 

 violet paper, the internal fpectrum was green, with 

 a reddifh-bluc halo. When the red internal paper 

 was two inches fquare, the internal fpectrum was 

 a deeper green, and the external one redder. When 

 the internal paper was fix inches fquare, the fpec- 

 trum of it became blue, and the fpectrum of the 

 external paper was red. 



f. When a fquare half-inch of blue paper was 

 laid on a fix-inch fquare of yellow paper, the fpec- 

 trum of the central paper in the clofed eye was 

 yellow, incircled with a blue halo. On looking 

 long on the meridian fun, the difk fades into a pale 

 blue furrounded with a whitifh halo. 



Thefe circumftances, though they very much 

 perplexed the experiments till they were invefti- 

 gated, admit of a fatisfa&ory explanation ; for while 

 the rays from the bright internal object in exp. a. 

 fall with their full force on the center of the retina, 

 and, by fatiguing that part of it, induce the reverfe 

 fpectrum, many fcattered rays, from the fame in- 

 ternal pink paper, fall on the more external parts 

 of the retina, but not in fuch quantity as to occa- 

 lion much fatigue, and hence induce the direct fpec- 

 trum of the pink colour in thofe parts of the eye. 

 The fame reverfe and direct fpectra occur from the 

 violet paper in exp. b. : and in exp. c. the fcattered 

 rays from the central pink paper produce a direcl fpec- 

 trum of this colour on the external parts of the eye, 

 while the fcattered rays from the external blue paper 

 produce a direct fpectrum of that colour on the cen- 



tral 



