452 OCULAR SPECTRA. SECT. XL. 5. i. 



through the eyelids, or by throwing the fpedrum on* white 

 paper, it becomes a reverfe fpedrum, as wiJl be explained be- 

 low. 



The other fenfes alfo retain for a time the impreflions that 

 have been made upon them, or the actions they have been ex- 

 cited into. So if a hard body is prefled upon the palm of the 

 hand, as is pradifed in tricks of legerdemain, it is not eafy to 

 diftinguim for a few feconds whether it remains or is removed ; 

 and taftes continue long to exift vividly in the mouth, as the 

 fmoke of tobacco, or the tafte of gentian, after the fapid mate- 

 rial is withdrawn. 



V. A quantity cfjlimulusfome'what greater than the lajl mentioned 

 excites the retina into fpafmodic aclion, ivhich ceafes and recurs 

 alternately. 



1. ON looking for a time on the fetting fun, fo as not great- 

 ly to fatigue the fight, a yellow fpeflrum is feen when the eyes 

 are clofed and covered, which continues for a time, and then 

 difappears and recurs repeatedly before it entirely vanifhes. 

 This yellow fpeftrum of the fun when the eyelids are opened 

 becomes blue ; and if it is made to fall on the green grafs, or on 

 other coloured objects, it varies its own colour by an intermix- 

 ture of theirs, as will be explained in another place. 



2. Pla.ce a lighted fpermaceti candle in the night about one 

 foot from your eye, and look fleadily on the centre of the flame, 

 till your eye becomes much more fatigued than in Seft. IV. 

 Exp. 3. ; and on clofing your eyes a reddifh fpectrum will be 

 perceived, which will ceafe and return alternately. 



The action of vomiting in like manner ceafes, and is renew- 

 ed by intervals, although the emetic drug is thrown up with the 

 firil effort ; fo after-pains continue fome time after parturition ; 

 and the alternate pulfations of the heart of a viper are renew- 

 ed for fome time after it is cleared from its blood. 



VI. OF REVERSE OCULAR SPECTRA. 



The retina, after having been excited into aclion by a Jlimulus fome- 

 what greater than the I aft mentioned \ falls into oppofite fpafmodic 

 aclion. 



THE actions of every part of animal bodies may be advanta- 

 geoufly compared with each other. This ftrict analogy con- 

 tributes much to the inveftigation of truth ; while thole loofer 

 analogies, which compare the phenomena of animal life with 



thofe 



