Sect. XL. 10.4. OCULAR SPECTRA. 463 



From hence it would feem, that the centre of the eye produ- 

 ces quicker remiffions of fpectra, owing perhaps to its greater 

 fenfibility ; that is to its more energetic exertions. Thefe re- 

 miflions of fpectra bear fome analogy to the tremors of the 

 hands, and palpitations of the heart, of weak people : and per- 

 haps a criterion of the ftrength of any mufcle or nerve may be 

 taken from the time it can be continued in exertion. 



4. Variation of fpeclra in refpeel to brilliancy ; the vifihUitj of the 



circulation of the blood in the eye. 



1. The meridian or evening light makes a difference in the 

 colours of fome fpeclra ; 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 be- 

 comes blue either a little later or earlier ; and white in the me- 

 ridian 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 confounded, if they 

 are made too foon after each other, as the remaining fpectrum 

 will mix with the new ones. This is a very troublefome cir- 

 cumftance to painters, who are obliged to look long upon the 

 fame colour ; and in particular to thofe whofe eyes, from natur- 

 al debility, cannot long continue the fame kind of exertion. 

 For the fame reafon, in making thefe experiments, the refult be- 

 comes much varied if the eyes, after viewing any object, are re- 

 moved on other objects for but an inftant of time, before we 

 clofe them to view the fpectrum ; for the light from the object, 

 of which we had only a tranfient view, in the very time of do- 

 ling our eyes acts as a ftimulus on the fatigued retina ; and for 

 a. time prevents the defired fpectrum from appearing, or mixes 

 its own fpectrum with it. Whence, after the eyelids are clofed, 

 either a dark field, or fome unexpected colours, are beheld for a 

 few feconds, before the defired fpectrum becomes diitinCtlyvifible. 



3. The length of time taken up in viewing an object, of 

 which we are to obferve the fpectrum, makes a great difference 

 in the appearance of the fpectrum, not only in its vivacity, but 

 in its colour ; as the direct fpeftrum of the central object, or of 

 the circumjacent ones, and alfo the reverfe fpeftra of both, with 

 their various combinations, as well as the time of their duration 

 in the eye, and of their remiiTions or alterations, depend upon 

 the degree of fatigue the retina is fubjected to. The Chevalier 

 d'Arcy conftru<ted a machine by which a coal of fire was 

 whirled round in the dark, and found, that when a luminous 



body 



