SECT. XL. 6. OCULAR SPECTRA, 619 



varies its own colour by an intermixture of theirs, 

 as will be explained in another place. 



2. Place a lighted fpermaceti candle in the night 

 about one foot from your eye, and lookfleadily on 

 the centre of the flame, till your eye becomes much 

 more fatigued than in Sect. IV. Exp. 3. ; and on 

 clofing your eyes a rcddifh fpectrum will be per- 

 ceived, which will ceafe and return alternately. 



The action of vomiting in like manner ceafes, and 

 is renewed by intervals, although the emetic drug 

 is thrown up with the firft effort : fo after-pains con- 

 tinue fome time after parturition; and the alternate 

 pulfations of the heart of a viper are renewed for 

 fome time after it is cleared from its blood. 



VI. OF REVERSE OCULAR SPECTRA. 



The retina, after having been excited into aftion by a 

 Jlimulus fomewhat greater than the lajl mentioned, 

 falls into oppofite fpafmodic aflion. 



THE actions of every part of animal bodies may 

 be advantageoufly compared with each other. This 

 ftrict analogy contributes much to the investigation 

 of truth ; while thofe loofer analogies, which com- 

 pare the phenomena of animal life with thofe of 

 chemiftry or mechanics, only ferve to miilead our 

 inquiries. 



When any of our larger mufcles have been in 

 long or in violent action, and their antagonifts have 

 been at the fame time extended,as foon as the adlion ot 

 the former ceafes, the limb is ftretched the contrary 

 way for our eafe, and a pandiculation or yawning 

 takes place. 



By the following obfervation it appears, that a 



iimilar circumftance obtains in the organ of vifion ; 



S s 2 after 



