WITHIN SOLID AND FLUID BODIES. 115 
Sir Joun 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 MN OP 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 AC 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 
ACB comparatively faint. When we view the bright blue stratum in the direction 
NM, 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 thestratum ; 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 
DE, so as to receive the beam A BED, we shall find that there is no peculiar 
dispersion, as Sir Joun HERscueEx observed, either at its surface of incidence or 
emergence. Hence he concluded that the epipolised beam A BE D “ 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 A BC, even when two inches long, and with a December sun, disperses 
the blue light, and the stratum behind the glass plate DE 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 Joun 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. 2G 
