SECT. XL. 10. OCULAR SPECTRA. 629 



the red filk, or the dired fpeclrum of all the rays 

 from the white paper, except the red ; for in truth 

 it is both. Hence we fee the reafon why it is not 

 eafy to gain a direct fpectrum of any coloured ob- 

 ject in the day -time, where there is much lateral 

 light, except of very bright objects, as of the fetting 

 fun, or by looking through an opake tube ; becaufe 

 the lateral external light falling alfo on the central 

 part of ihe retina, contributes to induce the reverfe 

 fpectrum, which is at the fame time the direct fpec- 

 trum of that lateral light, deducting only the colour 

 of the central object which we have been viewing. 

 And for the fame reafon, it is difficult to gain the 

 reverfe fpectrum, where there is no lateral light to 

 Contribute to its formation. Thus, in looking 

 through an opake tube on a yellow wall, and clofing 

 my eye, without admitting any lateral light, thefpec- 

 tra were all at firft yellow ; but at length changed 

 into blue. And on looking in the fame manner on 

 red paper, I did at length .get a green fpectrum ; but 

 they were all <at firft red ones : and the fame after 

 looking at a candle in the night. 



Ihe reveife fpectrum was formed with greater fa- 

 cility when the eye was thrown from the object on 

 a iheet of white paper, or when light was admitted 

 through the clofcd eyelids ; becaufe not only the 

 fatigued part of the retina was inclined fpomane- 

 ouily to fall into motions of a contrary direction ; 

 but being Rill fenfible to all other rays of light ex- 

 cept that with w : hich it was lately fatigued, was by 

 thefe rays flimulated at the fa ne time into thofe mo- 

 tions which form the reverie fpectrum. Hence, 

 when the reverfe fpecirum of any colour became 

 faint, it was wonderfully revived by admitting more 

 light through ihe eyelids, by removing the hand 

 from before them : and hence, on covering the clofed 

 eyelids, the fpectrum would often ceafe for a time, 

 till the retina became fenfible :Q. the flimulus of the 



fmaller 



