176 ANNUAL OF SCIENTIFIC DISCOVERT. 



end of the former. Whatever be the screen, the horizontal band is 

 always situated below the oblique, since there appears to be no excep- 

 tion to the law, that when the refrangibility of light is changed in this 

 manner it is always lowered. The general appearance of some highly 

 " sensitive " media in the invisible rays was then exhibited by means 

 of the flame of sulphur burning in oxygen, a source of these rays 

 which Dr. Faraday, had in some preliminary trials found very effica- 

 cious. The chief media used were articles made of glass colored by 

 uranium, and solutions of quinine, of horse-chestnut bark, and of the 

 seeds of the datura stramonium. A tall cylindrical jar filled with 

 water showed nothing remarkable ; but when a solution of horse- 

 chestnut bark was poured in, the descending fluid was strongly lumin- 

 ous. The experiment was varied by means of white paper on which 

 words had been written with a pretty strong solution of sulphate of 

 quinine, an alcoholic solution of the seeds of the datura stramonium, 

 and a purified aqueous solution of horse-chestnut bark. By gaslight 

 the letters were invisible ; but by the sulphur light, especially when 

 it had been transmitted through a blue glass, which transmits a much 

 larger proportion of the invisible than of the visible rays, the letters ap- 

 peared luminous, on a comparatively dark ground. A glass vessel 

 containing a thin sheet of a very weak solution of chromate of potash 

 * allowed the letters to be seen as well, or very nearly as well as before, 

 when it was interposed between the eye and the paper ; but when it 

 was interposed between the flame and the paper the letters wholly dis- 

 appeared, the medium being opaque with respect to the rays which 

 caused the letters to be luminous, but transparent with respect to the 

 rays which they emitted. It was then remarked what facilities are 

 thus afforded for the study of the invisible rays. When a pure spec- 

 trum is once formed, it is as easy to determine the mode of absorption 

 of an absorbing medium with respect to the visible rays. It is suffi- 

 cient to interpose the medium in the path of the incident rays, and to 

 notice the effect. Again, the effect of various flames, and other sources 

 of light on solutions of quinine, and on similar media, indicates the 

 richness or poverty of those sources with respect to the highly refrangi- 

 ble invisible rays. Thus the flames of alcohol, of hydrogen, &c., of 

 which the illuminating power is so feeble, were found to be very rich 

 in invisible rays. This was still more the case with a small electric 

 spark, while the spark from a Leyden jar was found to abound in rays 

 of excessively high refrangibility. These highly refrangible rays 

 were stopped by glass, but passed freely through quartz. These 

 results, and others leading to the same conclusion, had induced the 

 lecturer to order a complete train of quartz. A considerable portion 

 of this was finished before the end of last August, and was applied to 

 the examination of the solar spectrum. A spectrum was then obtained 

 extending beyond the visible spectrum that is, beyond the extreme 

 violet, to a distance at least double that of the formerly known 

 chemical spectrum. This new region was filled with fixed lines like 

 the regions previously known. But a spectrum far surpassing this 

 was obtained with the powerful electrical apparatus belonging to the 



