634 OCULAR SPECTRA. SECT; XL. 10. 



lour the external and internal areas blue, as in Fig. 

 4. ; after having looked about a minute on the cen- 

 tre of thefe circles, in a bright light, the fpectrum 

 of the external area appears firft in the clofed eye, 

 then ihe middle area, andlaftly the central one; and 

 then the central one difappears, and the others in 

 inverted order. If concentric circles of more colours 

 are added, it produces the beautiful ever changing 

 fpectrum in Sect. I. Exp. 2. 



From hence it would feern, that the centre of the 

 eye produces quicker remiffions of fpectra, owing 

 perhaps to its greater fenfibility ; that is, to its more 

 enei getic exertions. Thefe remiflious of fpectra bear 

 fome analogy to the tremors of the hands, and pal- 

 pitations of the heart, of weak people : and perhaps 

 a criterion of the ftrength of any mufcle or nerve 

 may be taken from the time it can be continued in 

 exertion. 



4. Variation offpe&ra in refpefl to brilliancy ; the vifi- 

 bility of the circulation of the blood in the eye. 



1. The meridian or evening light makes a differ- 

 ence in the colours of fome fpectra ; for as the fun 

 defcends, the red rays, which are lefs refrangible by 

 the convex atmofphere, abound in great quantity. 

 Whence the fpectrum of the light parts of a window 

 at this time, or early in the morning, is red ; and 

 becorhes blue either a little later or earlier ; and 

 white in the meridian day ; and is alfo variable from 

 the colour of the clouds or fky which are oppofed to 

 the window. 



2. All thefe experiments are liable to be con- 

 founded, if they are made too foon after each other, 

 as the remaining fpectrum will mix with the new 

 ones. This is a very troublefome circumftance to 

 painters, who are obliged to look long upon the fame 



colou; 



