NATURAL PHILOSOPHY. 175 



once the fundamental hypothesis was admitted, that he could not help 

 feeling strongly impressed that it would turn out to be true. Its 

 truth or fallacy was a question easily to be decided by experiment ; 

 the experiments were performed and resulted in its complete es- 

 tablishment. 



The lecturer then described what may be regarded as the funda- 

 mental experiment. A beam of sunlight was reflected horizontally 

 through a vertical slit into a darkened room, and a pure spectrum was 

 formed in the usual manner, namely, by transmitting the light through 

 a prism at the distance of several feet from the slit, and then through 

 a lens close to the prism. In the actual experiment, two or three 

 prisms were used, to produce a greater angular separation of the 

 colors. Instead of a screen, there was placed at the focus of the lens 

 a vessel containing a solution of sulphate of quinine. It was found 

 that the red, orange, &c., in fact, nearly the whole of the visible rays, 

 passed through the fluid as if it had been mere water. But on arriv- 

 ing about the middle of the violet, the path of the rays within the 

 fluid was marked by a sky-blue light, which emanated in all directions 

 from the fluid, as if the medium had been self-luminous. This blue 

 light continued throughout the region of the violet, and far beyond, 

 in the region of the invisible rays. The posterior surface of the 

 luminous portion of the fluid marked the distance to which the inci- 

 dent rays were able to penetrate into the medium before they were 

 exhausted. This distance, which at first exceeded the diameter of the 

 vessel, decreased with great rapidity, so that in the greater part of the 

 invisible region it amounted to only a very small fraction of an inch. 

 The fixed lines of the extreme violet, and of the more refrangible 

 invisible rays, were exhibited by dark planes interrupting the dis- 

 persed light. When a small portion of the incident spectrum was 

 isolated, by stopping the rest by a screen, and the corresponding beam 

 of blue dispersed light was refracted sideways by a prism held to the 

 eye, it was found to consist of light having various degrees of refrangi- 

 bility, with color corresponding, the more refrangible rays being more 

 abundant than the less refrangible. The nature of epipolized light is 

 now evident ; it is nothing but light from which the highly refrangible 

 invisible rays have been withdrawn by transmitting it through a solu- 

 tion of quinine, and does not differ from light from which those rays 

 have been withdrawn by any other means. The difference of nature 

 of the illumination produced by a change of refrangibility, or " true 

 internal dispersion," from that due to the mere scattering of light, may 

 be shown in a very instructive form by placing paper washed with 

 sulphate of quinine, or a screen of similar properties, so as to receive 

 a long narrow horizontal spectrum, and refacting this upwards by a 

 prism held to the eye. Were the luminous band formed on the paper 

 due merely to the scattering of the incident rays, it ought of course 

 to be thrown obliquely upwar/ls ; whereas it is actually decomposed 

 by the prism into two bands, one ascending obliquely, and consisting 

 of the usual colors of the spectrum in their natural order, the other 

 running horizontally, and extending far beyond the more refrangible 



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