SECT. XL. 10. 2. OCULAR SPECTRA. 4 j r; 



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

 reverfe fpe&rum of the central object, and the direct fpedrum 

 of the circumjacent objects, in proportion to their reipeftive 

 quantity and brilliancy. 



2. Variation and vivacity of the fpeclra occaftoned by extraneous 



light. 



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

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

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

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

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

 prifmatic colours. Now it muit 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 fpeftrum which is perceived, may either be 

 called the reverfe fpe&rum of the red fiik, or the direct fpec- 

 trum 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 fpedlrum 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 ; becaiife 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 direct fpeftrum of that lat- 

 eral 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 opaque 

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

 any lateral light, the fpectra 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 fpectrum ; but 

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

 candle in the night. 



The reverfe fpectrum was formed with greater facility when 

 the eye was thrown from the object on a fheet of white paper, 

 or when Lght was admitted through the clofed eyelids ; becaufe 

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

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

 ftill fcnfible 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 fpectrum* 



Hence, 



