150 SIR J. F. W. HERSCHEL ON THE 



less, or but little coloured, in hopes of discovering something which might elucidate 

 the subject. Strong alcohol, solution of corrosive sublimate, ammonia, &c. acted as 

 water ; allowing the blue film to be seen externally at a perpendicular incidence of 

 the visual ray to the surface of the liquid. With strong sulphuric acid, and with 

 muriate of lime so concentrated as to be syrupy, this was not possible, but the film 

 became visible, and of its full intensity, on moving the eye forward (i. e. towards the 

 incident light). When sulphate of manganese was used, its delicate pale rose-co- 

 lour no way prevented a fine exhibition of the blue film (a point to which I shall have 

 occasion to revert). On the other band, the lemon yellow-colour of nitrate of ura- 

 nium (a much fuller tint) materially enfeebled, though it did not prevent the forma- 

 tion of the film. This last effect did however appear to be produced by two liquids, 

 viz, pyroxylic spirit in a small degree, and oil of turpentine in a much greater ; the 

 effect in this case being very obviously much more than could justly be attributed to 

 a trifling tinge of yellow in the oil (which was not fresh), as I satisfied myself by a 

 comparative experiment with water purposely coloured to a similar tint of greater 

 intensity. Neither of these liquids however was found on trial in the test cylinder, 

 or otherwise, to possess in the smallest degree the property of epipolic dispersion; 

 nor have I found any other liquid which does so. 



Exp. 5. Among solids the only one I am acquainted with possessed of a simi- 

 lar property, is the green fluor of Alston Moor, which exhibits by superficial 

 dispersion a fine deep blue colour, very different from the inherent or absorptive 

 colour of the mineral. This is strictly an epipolic tint, as the following experiment 

 will show, and at the same time affords another, and not a little striking confirmation 

 of the general proposition announced in p. 148. I should premise that to see the 

 epipolic colour of the fluor in perfection, it must be laid on black velvet, or the re- 

 flexion of light from its posterior surfaces must be destroyed by roughening and 

 coating them with black sealing-wax. In this state, if exposed to daylight at a 

 window, and viewed through a blackened eye-tube, it is seen not as a green, but as a 

 fine deep blue crystal. 



If a piece of fluor so prepared be placed in water in a glass standing on black 

 velvet, the blue epipolic colour is seen greatly heightened. But if the water be 

 exchanged for a solution of quinine, this colour is completely destroyed and the 

 surface appears simply black. To make the experiment successfully, the greatest care 

 must be taken to cut off all lateral or reflected light. The arrangement I adopted 

 was, to coat a fluor as above described, and fastening it with black sealing-wax to 

 a wire, to lower it into the coated jar described in Exp. 4, filled alternately with a 

 solution of quinine and with pure water. Using the eye-tube for further precaution, 

 the destruction of the epipolic tint by the solution was quite as complete as if instead 

 of the fluor a test glass full of the quiniferous solution had been used. 



It would certainly appear from these experiments that the residual beam after un- 

 dergoing epipolic dispersion had lost some constituent portion, or otherwise under- 



