SECT. XL. 6. r. 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 vio- 

 lent aclion, and their antagonifts have been at the fame time 

 extended, as foon as the aftion of the former ceafes, the limb is 

 ftretched the contrary way for our eafe, and a pandiculation or 

 yawning takes place. 



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

 cumftance obtains in the organ of vifion ; after it has been fa- 

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

 pofire kind. 



1 . Place a piece of coloured filk, about an inch in diameter, 

 on a fheet of white paper, about half a yard from your eyes j 

 look fteadily upon it fo< a minu e ; then remove your eyes up- 

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

 of rhe form of the filk thus inflected, but of a colour nppofite 

 to it. A Ipeclrum nearly fimilar will appear if the eyeb 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 filk produced a green fpeftrum. 



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 fpera are the 

 reverfe of the colour^ which occafioned them, may be feen by 

 examining the third figjre in Sir Ilaac Newton's Optics, L. II. 

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

 tranfmitted violet ; thofe which reflected red, tranfmitted a blue 

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

 related. 



2. Thefe reverfe fpeftra 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 fpeclrum of red muft be fuch a green as would be pro- 

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

 evince this fal the following fatisfaftory 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. !2. 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 



