34* Da*. Darwin’s Experiments on the 
the candle two pens are feen. If the fight be unfteady at the 
time of beholding the fun, even though one eye only be ufed, 
many images of the fun will appear, or luminous lines, when the 
eye is clofed. And as fome parts of thefe will be more vivid than 
others, and fome parts of them will be produced nearer the 
center of the eye than others, thefe will difappear fooner than 
the others ; and hence the number and Ihape of thefe fpeftra 
of the fun will continually vary, as long as they exift. The 
caufe of fome being more vivid than others, is the unfteadinefs 
of the eye of the beholder, fo that fome parts of the retina 
have been longer expofed to the funbeams. That fome parts 
of a complicated fpeftrum fade and return before other parts of 
it, the following experiment evinces. Draw three concentric 
circles ; the external one an inch and a half in diameter, the 
middle one an inch, and the internal one half an inch ; colour 
the external and internal areas blue, and the remaining one 
yellow, as in fig. 4. ; after having looked about a minute on 
the center of thefe circles, in a bright light, the fpe&rum of 
the external area appears firft in the clofed eye, then the middle 
area, and laftly the central one; and then the central one 'dis- 
appears, and the others in inverted order. If concentric circles 
of more colours are added, it produces the beautiful ever chang- 
ing fpedtrum in fedt. I. exp. 2. 
From hence it would feem, that the center of the eye pro- 
duces quicker remiffions of fpe&ra, owing perhaps to its 
greater fenfibility ; that is, to its more energetic exertions. 
Thefe remiffions of fpedtra bear fome analogy to the tremors 
of the hands, and palpitations of the heart, of weak people ; 
and perhaps a criterion of the ftrength of any mufcle or nerve 
m^y be taken from the time it can be continued in exertion, 
$ 
iv* 
