OCULAR SPECTRA. SECT. XL. 4 



inflammation is excited, which terminates in gan- 

 grene or fever. 



IV. OF DIRECT OCULAR SPECTRA. 



A quantity of Jlimulus fomewhat greater than natural 

 excites the retina into fpafmodic adion, which ceafes 

 in> ajewfecondf. 



A certain duration and energy of the ilimulus 

 of light and colours excites the perfe6l a6tion of the 

 retina in viiion ; for very quick motions are imper- 

 ceptible to us, as well as very flow ones, as the 

 whirling of a top, or the fhadow on a fun-dial. So 

 perfe6t darknefs does not affedl the eye at all ; and 

 excels of light produces pain, not viiion. 



I. When a fire-coal is whirled round in the dark, 

 & lucid circle remains a confiderable time in die 

 eye ; and that with fo much vivacity of light, that 

 it is mi (taken for a continuance of the irritation of 

 the objeft. In the fame manner, when a fiery me- 

 t'Cor fhoots acrofe the night, it appears to leave a 

 Jong lucid train behind it, part of which, and per- 

 haps fometimes the whole, is owing to the conti- 

 nuance of the a6lion of the retina after having been 

 thus vividly excited. This is beautifully illuflrated 

 by the following experiment : fix a paper fail, three 

 or four inches in diameter, and made like that of 

 a fmoke jack, in a tube of pafteboard ; on looking 

 through the tube at a diftant profpedl, fome dis- 

 iornted parts of it will be feen through the narrow 

 intervals between the fails ; but as the fly begins to 

 fevolve, thefe intervals appear larger ; and when it 

 revolves quicker, the whole profpeft is feen quite 

 as diftinft as if nothing intervened, though lefs lu- 

 minous* 



2. Look 



