6i8 OCULAR SPECTRA. SECT. XL. 5 



not fo great as to produce any degree of uneafinef^. 

 in the organ of vifion ; which diftinguifhes therri 

 from the next clafs of ocular fpectra, which are the 

 confcquence of fatigue. Thefe direct fpectra are 

 beft obferved in fuch c ire urn fiances that no light, 

 but what comes from the object, can fall upon the 

 eye ; as in looking through a tube, of half a yard 

 long, and an inch wide, at a yellow paper on the 

 fide of a room, the direct fpectrum was eafily pro- 

 duced on clofing the eye without taking it from the 

 tube; but if the lateral light is admitted through 

 the eyelids, or by throwing the fpectvum on white 

 paper, it becomes a reverfe fpectrum, as will be 

 explained below. 



The other fenfes alfo retain for a time the impref- 

 fions that have been made upon them, or the ac- 

 tions they have been excited into. So if a hard 

 body is prefled upon the palm of the hand, as is 

 practifed in tricks of legerdemain, it is not eafy to 

 diftinguifh 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 of gentian, after the fapid material is withdrawn. 



V. A quantity of Jlimulus fomewhat greater than ikt 

 lajl mentioned excites the retina into ffafmodic aflion, 

 which ceafes and recurs alternately. 



I. ON looking for a time on the fetting fun, fo 

 as not greatly to fatigue the fight, a yellow fpectrum 

 is feen when the eyes are clofed and covered, which 

 continues for a time, and then difappears and re- 

 curs repeatedly before it entirely vanifhes. This 

 yellow fpectrum 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 



