Sect. XL. 6. i. OCULAR SPECTRA. 453 



thofe of chemiftry or mechanics, only ferve to miflead our in- 

 quiries. 



When any of our larger mufcles have been in long or in violent 

 action, and their antagonifts have been at the fame time extend- 

 ed, as foon as the action of the former ceafes, the limb is ftretch- 

 ed the contrary way for our eafe, and a pandiculation or yawn- 

 ing takes place. 



By the following obfervations it appears, that a fimilar cir- 

 cumltance obtains in the organ of vifion •, after it has been fa- 

 tigued by one kind of action, it fpontaneoufly falls into the op- 

 pofite kind. 



1. place a piece of coloured (ilk, about an inch in diameter, 

 on a meet of white paper, about half a yard from your eyes ; 

 look fteadily upon it for a minute ; then remove your eyes up- 

 on another part of the white paper, and a fpectrum will be feen 

 of the form of the (ilk thus infpected, but of a colour oppofite 

 to it. A fpectrum nearly fimilar will appear if the eyes are 

 clofed, and the eyelids (haded by approaching the hand near 

 them, fo as to permit fome, but to prevent too much light fall- 

 ing on them. 



Red (ilk produced a green fpectrum. 



Green produced a red one. 



Orange produced blue. 



Blue produced orange. 



Yellow produced violet. 



Violet produced yellow. 

 That in thefe experiments the colours of the fpedtra are the 

 reverfe of the colours which occafioned them, may be feen by 

 examining the third figure in Sir Ifaac Newton's Optics, L. II. 

 p. i. where thofe thin laminse of air, which reflected yellow, 

 tranftnitted violet 5 thofe which reflected red, tranfmitted a blue 

 green ; and fo of the reft, agreeing with the experiments above 

 related. 



2. Thefe reverfe fpectra are fimilar to a colour, formed by a 

 combination of all the primary colours except that with which 

 the eye has been fatigued in making the experiment : thus the 

 reverfe fpectrum of red muft be fuch a green as would be pro- 

 duced by a combination of all the other prifmatic colours. To 

 evince this fact the following fatisfactory experiment was made. 

 The prifmatic colours were laid on a circular pafteboard wheel, 

 about four inches in diameter, in the proportions defcribed in 

 Dr. Prieftley's hiftory of Light and Colours, pi. 12. fig. 83. ex- 

 cept that the red compartment was entirely left out, and the 

 others proportionably extended fo as to complete the circle. 

 Then, as the orange is a mixture of red and yellow, and as the 



violet 



