Page:Proceedings of the Royal Society of London Vol 1.djvu/50

Rh Considering this curious structure of the nose in an animal which lives in water, it is natural to conclude that nature has fitted it for discovering its prey in that element, by means of the sense of smell; and that for this purpose it is enabled to introduce this prominence into the small recesses in which its natural food is probably concealed.

In the ﬁrst part of this paper* the Doctor had proposed the seven following points which he meant to elucidate in this inquiry. 1. That heat, both solar and terrestrial, is a sensation occasioned by rays emanating from candent substances. 2. That these rays are subject to the laws of reﬂection. 3. That they are refrangible. 4. That they are of different refrangibility. 5. That they are liable to be detained in their passages through other bodies. 6. That they are also liable to be scattered on rough surfaces. And lastly, he proposed to ascertain whether in a certain degree of energy these rays may not have or acquire a power of illuminating objects. The three former points have been considered in the ﬁrst, and the four last are the subjects of the present part of the paper.

Concerning the different refrangibility of the rays of heat, being the subject of the fourth article, we ﬁnd that in refracting the rays of the sun by a prism, two distinct spectra may be said to be produced, the one of light, and the other of heat, the latter being distinctly observable by means of thermometers. These two spectra the Doctor has found means to represent by a ﬁgure, in which the length of the luminous or coloured spectrum, being represented by a line on which are raised ordinates proportionate to the quantity of illumination of each coloured ray, the curve joining these ordinates, together with this base line, inclose an area which may be said to represent the extent and intensity of the coloured rays. Adopting now another base line of the length of the range of the refracted rays of heat, one extremity of which is found to coincide with the termination of the coloured spectrum at the outward edge of the violet ray, and the other to project beyond the opposite termination at the red ray, which makes this line longer than the other, in the proportion of nearly 5¼ to 3. Ordinates are here in like manner applied according to the different degrees of intensity of heat indicated by accurate thermometers, and thus another area is produced, which represents the spectrum of heat both as to extent and intensity. On inspecting these ﬁgures, parts of which coincide, but other parts considerably deviate from each other, we ﬁnd that the coloured and the heating rays differ widely, both in their mean refrangibility and