SECT. XL. 10. 4. OCULAR SPECTRA. 46.3 



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

 ces quicker remiflions of fpe&ra, owing perhaps to its greater 

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

 miflions of fpeclra 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 offpeclra in refpeft to brilliancy ; the vijibility of the 

 circulation of the blood in the eye. 



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

 colours of fome fpeftra ; for as the fun defcends, the red rays-, 

 which are lefs refrangible by the convex atmofphere, abound in 

 great quantity. Whence the fpeclrum 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 (ky 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 fpeftrum 

 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 but for an inftant of time, before we 

 clofe them to view the fpeftrum ; for the light from the objeft, 

 of which we had only a tranfien' view, in the very time of clo- 

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

 a time prevents the defired fpeclrum from appearing, or mixes 

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

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

 fewfeconds,before the defired fpectrum becomes diftinUy vifible. 



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

 which we are to obferve the fpe&rum, makes a great difference 

 in the appearance of the fpe&rum, 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 fpectra of both, with 

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

 in the eye. and of their remiflions or alterations, depend upon 

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

 d'Arcy conirrucic-f! a machine by which a coal of fire was 

 thirled 'round in the dark, and found, that when a luminous 



body 



