456 OCULAR SPECTR A. SECT. XL. 9. i . 



this phenomenon j and from hence we may learn the danger to 

 the eye by infpecting very luminous objeftstoo long a time. 



IX. A quantity of Jlimulus greater than the preceding induces a tem- 

 porary paralyfis of the organ of vi/ion. 



1. PLACE a circular piece of bright red filk, about half an inch 

 in diameter, on the middle of a fhset of white paper , lay them 

 on the floor in a bright funfhine, and fixing your eyes ileadily 

 on the centre of the red circle, for three or four minutes, at the 

 diftance of four or fix feet from the objedt, the red filk will grad- 

 ually become paler, and finally ceafe to appear red at all. 



2. Similar to thefe are many other animal fafts ; as purges, 

 opiates, and even poiibns, and contagious matter, ceafe to ftim- 

 ulate our fyftem, after we have been habituated to their ufe. So 

 fome people lleep undifturbed by a clock, or even by a forge 

 hammer in their neighbourhood : and not only continued irrita- 

 tions, but violent exertions of any kind, are fucceeded by tempo- 

 rary paralyfis. The arm drops down after violent aclion, and 

 continues for a time ufelefs ; and it is probable, that thofe who 

 have periihed fuddenly in fwimming, or in fcating on the ice, 

 have owed their deaths to the paralyfis, or extreme fatigue, 

 which fucceeds every violent and continued exertion. 



X. MISCELLANEOUS REMARKS. 



THERE were fome circumftances occurred in making thefe ex- 

 periments, which were liable to alter the refults of them, and 

 which I (hall here mention for the affitiance of others, who may 

 wifh to repeat them. 



I. Of dire ft and inverfe fpeclra exijling at the fame time ; of recipro- 

 cal direft fpeftra ; of a combination of dire El and inverfe fpeclra ; 

 of a fpeclral halo ; rules to pre- determine the colours oj fpeclra. 



a. When an area, about fix inches fquare, of bright pink In- 

 dian paper, had been viewed on an area, about a foot fquare, of 

 white writing paper, the internal fpeclrum in the clofed eye was 

 green, being the reverie fpe&rum of the pink paper ; and the 

 external fpeclrum was pink, being the direft fpeftrum of the 

 pink paper. The fame circumftance happened when the inter- 

 nal area was white and external one pink ; that is, the internal 

 fpeclrum was pink, and the external one green. Ail the fame 

 appearances oc cured when the pink paper was laid on a black 

 hat. 



b. When 



