Page:Proceedings of the Royal Society of London Vol 1.djvu/41

Rh the yellow; but that, from the full deep green, the illuminating power decreases very sensibly, the blue being nearly upon a par with the red, while the violet is still more, and the purple the most deﬁcient of any.

From a collective view and a due contemplation of these well established facts, it is inferred, that perhaps they may lead to the discovery of different chemical properties in the different coloured rays; and that the various degrees of heat in different coloured ﬂames will probably be fully explained on the principles that may be deduced from them.

Here follows a section in which the author advances a position, that radiant heat is of different refrangibility, and that it is subject to the laws of the dispersion arising from this different refrangibility. We perceive from the drift of his arguments here advanced, that he inclines in favour of a distinction between caloriﬁc rays and caloriﬁc light, both being liable to refraction, but under different angles, and hence probably' possessed of different momenta. This, if admitted, will apply to the invisible _heat of red-hot iron gradually cooled till it ceases to shine, and likewise affords a solution of the reﬂection of invisible heat by concave mirrors.

We come next to an application of the result of the foregoing observations to the method of viewing the sun advantageously, with telescopes of large apertures and high magnifying powers. It is well known, that even with the naked eye, and much less with telescopes of large apertures, the sun cannot be viewed without a darkening apparatus. Dr. Herschel had repeatedly used red glasses for this purpose; but always found, that though the lustre was thereby sufficiently abated. yet they did not prevent an irritation on the eye, which was manifestly the effect of heat. This induced him to make experiments with glasses of various other colours, the result of which was, that, as might have been inferred from the above investigation, dark-green glasses are the most efficacious for intercepting the red or more caloriﬁc rays, and will therefore answer one of the purposes of the darkening apparatus. But as in viewing the sun we have besides the heat also its splendour to contend with, further trials were made to obviate this inconvenience: and for this purpose, common smoked glasses were found the most efficacious. Some directions are here given for smoking glasses uniformly, and in the most convenient manner.

From a series of telescopic experiments respecting this darkening apparatus. which concludes the paper, we learn, that as the heat will often crack the glasses when placed at or near the focus of the pencils of rays, a safer and more advantageous way of applying them is to place them before and near the small Speculum, or immediately behind the second eye-glass. In this last position a single dark-green glass was found of great utility; but a deep blue glass, with a blueish-green smoked one upon it, was still preferable: the sun appeared of a whiter colour than with any other composition, and the sensation of heat was by no means inconvenient.