ON THE CHEMICAL ACTION OF THE SOLAR RADIATIONS. 75 



elongated oval is well made out. This effect is pro- Fig- 53. 

 duced by the yellow and the lower green rays, and occupies 

 a space of about "30 ; then a fainter impression is visible on 

 the paper, and over the space upon which the blue and in- 

 digo falls there is an enlargement in width of the image, 

 and the colour is a pure brown. The action is continued - 

 over the violet and lavender spaces as an interior dark oval, 

 and it extends in diminished intensity to 1*50, when it sud- 

 denly ceases. 



(105) 78. Yellow, by Coke. — The entire length of the 

 spectrum is scarcely at all reduced, but the blue and violet 

 rays blend, forming one band of a faint reddish blue ; the 



green rays are somewhat lengthened ; the yellow rays are g^ 



mingled with the orange. 



(106) 78. The difference between this and the former spectrum (104) is 

 rather in degree. The image is as nearly as possible the same in all its 

 parts, but that the chemical action extends to the lower point ; it can indeed 

 be traced down into the orange ray. 



(107) 79. Uranium glass, Canary -yellow. — This is the peculiar yel- 

 low glass which is employed in the manufacture of toilet bottles and other 

 ornamental articles, which transmits a pale yellow light, and disperses an 

 unusual green light. Upon this quality depends its extensive use in orna- 

 mental glass manufacture. In my experiments I have employed a slab of this 

 glass, the thickness of which is 1 inch, its width 2^ inches, and its length 

 4^ inches ; and independently of some striae, the glass was of a pretty uni- 

 form character throughout. This slab enabled me to operate respectively 

 through the several thicknesses of I, or 2^^ or 4^^ inches, and thus to deter- 

 mine with very great exactness the thickness of this medium through which 

 the chemical rays would pass. If a block of this glass, which is a canary- 

 yellow when we look through it, is placed upon a piece of black velvet, and 

 we look at it, it appears of a fine yellowish green colour ; this green light 

 wanting, however, transparency, and exhibiting more the character of a 

 gleam of monochromatic light piercing through a mist. 



If we throw upon the face of this glass a condensed pure spectrum, and 

 look through the sides of it, so as to observe the passage of the rays, its 

 powers of internal dispersion become distinctly visible. From the fixed line 

 b we find this dispersion commences, but few of these rays are enabled to 

 penetrate through the 1-inch thickness of the glass. A little above F a 

 minimum point is very observable, and from this point the dispersion of the 

 rays becomes very decided ; and some of these green rays, when the light is 

 good, penetrate the glass. This green-dispersed light is visible for a con- 

 siderable distance beyond the ordinary spectrum ; the entire space which has 

 usually been designated as the invisible chemical rays, is rendered luminous. 

 [For a more detailed examination of the optical properties of this glass, I 

 must refer to Professor Stokes's Memoir ' On the Change of Refrangibility 

 of Light,' in the Philosophical Transactions for 1852.] 



The ordinary spectrum which permeates this glass is but slightly altered in 

 its character, the condition of the rays after having undergone absorption by 

 this medium being as follows : — Beyond the green ray appears a band of a 

 brownish hue, from the mixture of red with blue ; then the blue appears 

 again with considerable brightness. On looking at the coloured fringes pro- 

 duced by the prism, and interposing the uranium glass, it is evident that both 

 blue and violet rays do permeate. 



(108) 79. The chemical effect produced by the solar spectrum after it has 



