On the /fff<;fl!ons and Properiks if Li^htt 1 47 



n. 



Farther Experiments and Obfervaihns on the AffeBlons and Properties of Light. By Henrt 



Brougham, jun. Ejq* 



H 



A VIN G laid before the Royal Society an account of a courfe of experiments f on 

 lightj in which I had been enji;aged, and alfo of the conclufions which thefe experiments' 

 had taught me to draw ; I proceed in the following paper to relate the continuation of my 

 obfervations ; which, I hope, may not prove wholly uninterefting to fuch as honoured the 

 former part with their attention. I am frrll to unfold a new and, I think, curious pro- 

 perty of light, that may be indeed reckoned fourfold, as it holds, like the reft, equally with 

 refpe<fl to refraction, reflexion, inflexion, and deflexion ; thus preferving entire the fame 

 beautiful analogy in thefe four operations, which we have hitherto remarked. I fhall thea 

 confider feveral phenomena connedled either with this, or with the properties before de- 

 fcribed, and of which they afford fome flriking confirmations. 



Ohfervatton i.— THE fun fhining ftrongly into my darkened chamber, T placed, at a 

 fmall hole in the window- fliut, a prifm, with its refrafting angle (of 65°) upwards, fo 

 jhat the fpedlrum was caft on a chart placed at right angles to the incident rays, and four 

 feet from the prifm. 



In the rays parallel to the chart, and two feet from it, I placed a pin, whofe diameter 

 was yg of an inch, and fixed it fo that the axis of its fliadow on the fpecflrum might be 

 parallel to the fides of the fpeflrum A fet of images by reflexion was formed (fimilar to 

 thofe defcribed above %), all inclining to the violet ; but what I chiefly attended to at pre- 

 fent was their fliape. 1 had always obferved that the part formed out of the red-making 

 rays was broadelt, and that the other parts diminilhed in breadth regularly towards the 

 violet. I now delineated one or two, at about three inches from the (hadow ; and though 

 (from the pin's irregularities) the fides were by no means fmooth, yet the general (hapc 

 was in every pin, and with every prifm ufed, nearly as reprefented in fig. i. Plate VIE. 

 divided in the direction R A, according to the colours of the fpedrum in which they were 

 formed ; R O B A was red, and the broadeft ; that is, R A was broader than O B, the 

 confines of the red and orange; and G DE V was the violet, narrowed of all. 



Obfervation 2. — Between the pin and the prifm, Vo °f ^" m<:-\\ from the pin, was placed 

 a fcreen, through a fmall hole in which, of twice the pin's diameter, the rays of the fpec- 

 trum paflfed, and were reflefted into images by the pin ; thefe were pretty diftindl and 

 well defined, when received on a chart half afoot from the pin. They w/ere oblong, having 

 parallel fides and confufed ends; they were wholly of the colour whofe rays fell on the 

 jHii, unlefs when the white, mixed with thofe at the confines of the yellow and green, 



• PUilofophical Tranfaftions, 1797. 



f See Philof. Journal i. 551. 585. 



% PhLlofophical Tranfaftions for 1796, page 240, or Philofophical Journal i. 557. 



U a cauled 



