Fraunhofer, in his original measurements of the fixed lines of the spectrum, noted the thermometer, and those who since his time have occupied themselves with the determination of refractive indices, must have frequently noticed that changes of temperature affect the amount of refraction; yet of the many observations now on record, few have affixed to them the temperature at which they were made. Baden Powell has drawn out some refractive indices of the same substance taken at different temperatures, but remarks that the whole subject, both as to the facts of the case and as to their bearing on the theoretical formulæ, remains to be investigated. Among his beautiful researches on light, M. Jamin has recently published one or two experiments on refraction, heat, and density; but, with this exception, he has left untouched the great question of the influence of temperature. Some years ago Baden Powell kindly lent his instrument to one of us for the purpose of investigating this question. It is described and figured in the Report of the British Association for 1839. It consists essentially of a small telescope moving on a circle graduated so finely, that by means of a vernier and magnifying glass it may be read to 10"; to ensure accuracy, there is a vernier at each end of the moveable arm: in front of the telescope are appliances for fixing any solid prism, or a hollow prism capable of containing liquids, in such a position that there may be seen through it a ray of light proceeding from a distant slit. The telescope may be moved till a fine wire fastened in the field of view is brought to coincide with any of Fraunhofer’s lines the prismatic spectrum thus obtained, and the angle indicated by the position of the telescope on the graduated circle may he read off. In our recent joint research, as we desired measure the positions of the extreme lines A and H, it was necessary to work with the full light of the sun; its mirrored image, therefore, was kept behind the narrow slit by a heliostat, and the eye was protected from the glare of the yellow ray by means of a cobalt-blue glass.
XXXVI. On the influence of temperature on the refraction of light