l40 PHYSICAL OPTICS. 



he imagined. The light refle61ed by a plate of air, at anic 

 thickncfs nearly correfponding to the 11th red, appears to 

 the eye to be very nearly white ; but, under Favourable cir- 

 cumftances, the 11th red and the neighbouring colours may 

 /till be diftinguiflied. The light of fome kinds of coloured 

 glafs is pure red ; that of others, red with a little green : 

 fome Intercept all the light, except the extreme red and the 

 blue. In the blue light of a candle, expanded by the prifm, 

 the portions of each colour appear to be narrower, and the 

 intervening dark fpaces wider, than in the analogous fpedrum 

 derived from the light refle^led from a thin plate. The light 

 of burning alcohol appears to be green and violet only. The 

 pink dye fold in the (liops, which is a preparation of the car- 

 thamus, affords a good fpecimen of a yellow green light regu- 

 larly refle(5ted, and a crimfon probably produced by tranfmiffion. 



Vf. EXPERIMENT ON THE DARK RAYS OF RITTER. 



Dark rays of Exper, 6. The exigence of folar rays accompanying light, 



ifoT^^Dr^'^^ more refrangible than the violet rays, and cognifable by their 

 WoUailon. chemical effeds, was firtl afcertalned by Mr. Ritter : but Dr. 



Woilaflon made the fame experiments a very fhort time after- 

 wards, without having been informed of what hsc] been done 

 on theContinent. Thefe rays appear to extend beyond the violet 

 rays of the prifmatic fpeclrum, through a fpace nearly equal ta 

 that which is occupied by the violet. lu order to complete the 

 comparlfon of their properties with thofeof vifible light, T was 

 They are re- defirous of examining the effect of their reOedion from a thin 

 thin platr^' air plal^^ ^^^ air, capable of producing the well known ring^ of co, 

 >^ith the fame lours. For this purpofe, I formed an image of the rings, by 

 modification of ^p ^^^^ j-^l^^ mlcrofcope, with the apparatu?^ which i 



rings as v ft bis ' ' ^ 



light IS. have defcribed in the Journals of the Royal Inftitution^ and I 



threw this image; on paper dipped In a folution of nitrate of 

 filver, placed at the diftance of about nine inches from the 

 microfcope. In the courfe of an hour,, portions of three dark 

 rings were very dlfh*n6tly vifihle, mucli fjnaller than the bright- 

 ell rings of the coloured image, and coinciding very nearly, 

 in their diaienfions, with the rings of violet light that appeared; 

 upon the interpofition of violet glafs. I thought the dark rings 

 were a little fma:ller than the violet rings, but the difference 

 vvas not futficiently great to be accurately afcerlained ; it might 

 be as much as ^^ or ^^ oi" the diametwj§, but not greater. It 



is 



