4s6 OCULAR SPECTRA. Sect. XL. 9. u 



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

 the eye by infpecting very luminous objects too long a time. 



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

 porary paralyfis of the organ of vifton. 



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

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

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

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

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

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



2. Similar to thefe are many other animal facts j as purges, 

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

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

 fome people fleep 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 action, and 

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

 have perifhed fuddcnly 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 fhall here mention for the affiftance of others, who may 

 wifh to repeat them. 



I. Of direct and inverfe fpeclra exiting at the fame time; of recipro- 

 cal direct fpetlra ; of a combination of direct and inverfe fpetlra ,* 

 cf a fpeclral halo ; rules topre-determine the colours of 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 fpectrum in the clofed eye was 

 green, being the reverfc fpectrum of the pink paper ; and the 

 external fpectrum was pink, being the direct: fpectrum of the 

 pink paper. The fame circumftance happened when the inter- 

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

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

 appearances occurred when the pink paper was laid on a black 



hac. 



b. When 



