ocular Spetira of Light and Colours * 337 
11. Variation and vivacity of the fpeffira occajioned by extraneous 
light. 
The reverfe fpedtrum, 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 : fo the reverfe fp eel rum of red is fuch 
a green as would be produced by a combination of ail the Other 
prifmatic colours. Now it muft be obferved, that this reverfe 
fpedlrum of red is therefore the direct fpedtrum of a combina- 
tion of all the other prifmatic colours, except the red ; whence, 
on removing the eye from a piece of red filk to a fheet of 
white paper, the green fpedtrum, which is perceived, may 
either be called the reverfe fpedtrum of the red filk, or the 
diredt fpedtrum 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 diredt fpedtrum of any coloured objedt 
in the day-time, where there is much lateral light, except of 
very bright objedts, as of the fetting fun, or by looking 
through an opake tube; becaufe the lateral external light 
falling alfo on the central part of the retina, contributes to 
induce the reverfe fpedtrum, which is at the fame time the 
diredt fpedtrum of that lateral light, deducting only the colour 
of the central objedt which we have been viewing. And for the 
fame reafon, it is difficult to gain the reverfe fpedtrum, where 
there is no lateral light to contribute to its formation. Thus, 
in looking through an opake tube on a yellow wall, and doling 
my eye, without admitting any lateral light, the fpedtra were 
all at fir ft yellow ; but at length changed into blue. And on 
looking in the fame manner on red paper, I did at length get a 
Vol.LXXVI. Y y green 
