WITHIN SOLID AND FLUID BODIES. 115' 



Sir John has clearly shewn, that the light is dispersed outwards as well as 

 laterally ; but as he was conversant only with the phenomena of a narrow blue 

 line, and had not seen the blue cone of rays dispersed from the cone of condensed 

 light, he could not be aware of the changes which take place in its colour while 

 the eye passes from the azimuth of 90° to that of 100°. 



These changes are very decided, and will be understood from fig. 2 (Plate V.), 

 in which M N P is a horizontal section of the vessel containing the solution ; R R' 

 a beam of solar light, incident upon an achromatic lens L L, and condensed into the 

 luminous cone A C B. Now, the blue colour produced by the first stratum, next to 

 the side A B, is exceedingly strong, and that which occupies the rest of the cone 

 A C B comparatively faint. When we view the bright blue stratum in the direction 

 N M, or in the azimuth of 90°, the tint is very brilliant, because the eye receives all 

 the blue rays dispersed by the whole length A B of the stratum ; whereas, when we 

 view it in the direction R' C, in the azimuth of 0°, we only see the tint correspond- 

 ing to the thickness of the stratum. The tint, however, is, in reality, a maximum 

 in the azimuth of 0°, and gradually diminishes till it ceases in the azimuth of 180°, 

 or in the direction C R'. 



If we now immerse in the fluid a plate of colourless glass, whose section is 

 D E, so as to receive the beam ABED, we shall find that there is no peculiar 

 dispersion, as Sir John Herschel observed, either at its surface of incidence or 

 emergence. Hence he concluded that the epipolised beam ABED "is incapable 

 of undergoing farther epipolic dispersion ;" and that having thus " lost a property 

 which it originally possessed, it could not, therefore, be considered qualitatively as 

 the same light." 



Now, in using a condensed beam of light, as we have done, we find that the 

 whole cone ABC, even when two inches long, and with a December sun, disperses 

 the blue light, and the stratum behind the glass plate D E nearly as much as 

 the stratum before it. In fluor-spar, and in the other fluids I have mentioned, 

 this is still more strikingly the case,* and hence neither of the conclusions drawn 

 by Sir John Herschel are admissible. 



The following appear to me to be the deductions which the experiments 

 actually authorize : — 



1. A beam of light which has suffered dispersion by the action of a solid or 

 fluid body, (that is an epipolised beam) is capable of further undergoing epipolic 



* In one of these experiments a piece of green fluor, from Alston Moor, when immersed in the 

 quiniferous solution, dispersed a fine violet blue light, at the distance of three-fourths of an inch from 

 its surface. In another experiment, a beam of light that had been dispersed in the solution of qui- 

 nine, again suffered dispersion at two inches distance from the surface of a piece of Derbyshire fluor. 



A beam of light that has passed through the Esculine solution disperses blue light, but not co- 

 piously, when transmitted through the quinine solution ; but the beam that has passed through qui- 

 nine is copiously dispersed when transmitted through Esculine. 



VOL. XVI. PART II. 2 G 



