72 REPORT 1853. 



visible when the exposure to the spectrum has been prolonged, or the sun- 

 shine is very intense, and the atmosphere clear, after rain. 



(88) 68. a. Orange, Silver. — All the more refrangible ordinary rays 

 are very decidedly obliterated, and even the green somewhat shortened ; but 

 in the place of the blue and violet rays there is observable some red. The 

 yellow and orange are considerably reduced, the red standing out in great 

 brilliancy. 



(89) 68 a. A very singular result is obtained when the Fig- 45. 

 prismatic rays are subjected to the absorptive action of this ^ 

 medium. A faint spot makes its appearance in the yellow 



space, and in the point of maximum luminous intensity. No 



other action than this occurs within the limits of the visible £L 



spectrum ; but about -40 beyond a, a yet fainter spot of 



chemical action makes its appearance. Thus we have in 



this example evidence of two sets of chemical rays which 



have a very much greater penetrating power, relative at least 



to the yellow media we have been examining, than any of ^ 



the others situated in those parts of the spectrum which are w 



usually referred to as possessing the greatest chemical ^' 



power. Other examples of a similar description will be 



noticed. 



(90). Yellow, hy Carbon. — This glass, which is of a brownish colour, 

 and without much brilliancy, allows the free permeation of all the rays below 

 H. When the sun has been very brilliant, a slight shade of violet is visible 

 beyond the line H. The red of the violet is, however, nearly obliterated. 



This yellow medium gives a very decided and in- 

 tense chemical spectrum. The action commences at '25 "S- 46. 

 above a', and continues of an olive-brown colour to "50, 

 the oval formed at first gradually passing into a band. 

 Then a larger oval is formed, which extends to 1"50, and 

 sometimes still further. The overlapping of these, as pre- 

 viously noticed, is very apparent in this spectrum, and the 

 colours of the upper oval and the lower prolonged space 

 were as different as any which have yet been noticed. 

 There were also some evidences of those internal actions 

 which have been previously observed, but there was much 

 uniformity in the colours and characters of the inner and 

 outer images. 



(91) 68. Orange-coloured glass, Silver. — Possess- a^ 



ing in a remarkable degree the false dispersion observed by 

 Mr. Stokes. It reflects from one side, when placed on a piece of black 

 velvet, a peculiar bluish-green light; or when placed on a sheet of white 

 paper, the scattered light partakes of that mixture of blue and brown which 

 is ordinarily distinguished as a puce. 



Of this variety of glass, Mr. Stokes makes the following remarks in his 

 memoir ' On the Change of Refrangibility of Light': — "Orange-coloured 

 glasses are frequently met with which reflect from one side, or rather scatter 

 in all directions, a copious light of a bluish-green colour, quite different from 

 the transmitted tint. In such cases the body of the glass is colourless, and 

 the colouring matter is contained in a very thin layer on one face of the plate." 



This is not always the case ; in the glass with which the present experi- 

 ment wa"? made the colouring matter, silver, is diffused throughout the mass. 

 The peculiarity in question is produced on one surface by exposing it to the 

 influence of the flame of burning wood. 



Mr. Stokes continues : " As this phenomenon was supposed by Sir John 



I 



