ocular SpeSlra of bight and Colours * 341 
truip, they mix with it, their weaker ftimulus being infuffi- 
cient to induce the reverfe fpe&rum* 
in. Variation of fpedlra in refpedi to number and figure and 
remifjion . 
When we look long and attentively at any object, the eye 
cannot always be kept intirely motionlefs ; hence, on infpe£t~ 
ing a circular area of red filk placed on white paper, a lucid 
crefcent or edge is feen to librate on one fide or other of the red 
circle : for the exterior parts of the retina feme times falling on 
the edge of the central filk, and fometimes on the white paper, 
are lefs fatigued with red light than the central part of the re*- 
tina, which is conftantly expofed to it; and therefore, when 
they fall on the edge of the red filk, they perceive it more 
vividly. Afterwards, when the eye becomes fatigued, a green 
fpedlrumin the form of a crefcent is feen to librate on one fide 
or other of the central circle, as by the unfteadinefs of the 
eye a part of the fatigued retina falls on the white paper ; and 
as by the increafing fatigue of the eye the central part of the 
filk appears paler, the edge on which the un fatigued part of 
the retina occafiorially falls will appear of a deeper red than 
the original filk, becaufe it is compared with the pale internal 
part of it. M. be Buffon in making this experiment obferved, 
that the red edge of the filk w r as not only deeper coloured than 
the original filk ; but, on his retreating a little from it, it be- 
came oblong, and at length divided into two, which mu ft have 
been owing to a change of the angle of the two optic axifes 
with the new diftance he obferved it at. Thus, if a pen is 
held up before a diftant candle, when we look intenfely at the 
pen two candles are feen behind it \ when we look intenfely at 
7 the 
