Sect. XL. i®. 2. OCULAR SPECTRA. 459 



object. 4. with lateral light the colour is compounded of the 

 reverfe fpeclrum of the central object, and the direct fpeclrum 

 of the circumjacent objects, in proportion to their refpective 

 quantity and brilliancy. 



2. Variation and vivacity of the fpeclra occafioned by extraneous 



light. 



The reverfe fpeclrum, as has been before explained, is fimi- 

 lar to a colour, formed by a combination of all the primary 

 colours, except that with which the eye has been fatigued in 

 making the experiment ; Jo the reverfe fpeclrum of red is fuch 

 a green as would be produced by a combination of all the other 

 prifmatic colours. Now it muft be obferved, that this reverfe 

 fpeclrum of red is therefore the direct fpeclrum of a combina- 

 tion of all the other prifmatic colours, except the red ; whence, 

 on removing the eye from a piece of red (ilk to a fheet of white 

 paper, the green fpeclrum which is perceived, may either be 

 called the reverfe fpeclrum of the red filk, or the direcl fpec- 

 lrum 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 direcl fpeclrum of any coloured object 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 opaque 

 tube ; becaufe the lateral external light falling alfo on the cen- 

 tral part of the retina, contributes to induce the reverfe fpec- 

 trum, which is at the fame time the direcl fpeclrum of that lat- 

 eral light, deducting only the colour of the central objecl which 

 we have been viewing. And for the fame reafon, it is difficult 

 to gain the reverfe fpeclrum, where there is no lateral light to 

 contribute to its formation. Thus, in looking through an opaque 

 tube on a yellow wall, and clofing my eye, without admitting 

 any lateral light, the fpeclra were all at firft yellow ; but at 

 length changed into blue. And on looking in the fame man- 

 ner on red paper, I did at length get a green fpeclrum \ but 

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

 candle in the night. 



The reverfe fpeclrum was formed with greater facility when 

 the eye was thrown from the objecl on a (heet of white paper, 

 or when light was admitted through the clofed eyelids ; becaufe 

 not only the fatigued part of the retina was inclined fppntane- 

 oufly to fall into motions of a contrary direction ; but being 

 ftill fenfible to all other rays of light except that with which it- 

 was lately fatigued, was by thefe rays ftimulated at the fame 

 time into thofe motions which form the reverfe fpeclrum. 



Hence, 



