[B] I am not aware whether in his own country, or in any other, a recognition at all commensurate with the value of the performance has followed Schwerd's admirable essay entitled 'The Phenomena of Diffraction deduced from the Theory of Undulation.' [C] I think, however, that the strong irradiation from the glistening sides of the twigs and branches must also contribute to the result. [Sidenote: RADIANT HEAT.] (2.) Thus, then, we have been led from Sound to Light, and light now in its turn will lead us to _Radiant Heat_; for in the order in which they are here mentioned the conviction arose that they are all three different kinds of motion. It has been said that the beams of the sun consist of rays of different colours, but this is not a complete statement of the case. The sun emits a multitude of rays which are perfectly non-luminous; and the same is true, in a still greater degree, of our artificial sources of illumination. Measured by the quantity of heat which they produce, 90 per cent. of the rays emanating from a flame of oil are obscure; while 99 out of every 100 of those which emanate from an alcohol flame are of the same description.[A] [Sidenote: OBSCURE RAYS.] In fact, the visible solar spectrum simply embraces an interval of rays of which the eye is formed to take cognizance, but it by no means marks the limits of solar action. Beyond the violet end of the spectrum we have obscure rays capable of producing chemical changes, and beyond the red we have rays possessing a high heating power, but incapable of exciting the impression of light. This latter fact was first established by Sir William Herschel, and it has been amply corroborated since. The belief now universally prevalent is, that the rays of heat differ from the rays of light simply as one colour differs from another. As the waves which produce red are longer than those which produce yellow, so the waves which produce this obscure heat are longer than those which produce red. In fact, it may be shown that the longest waves never reach the retina at all; they are completely absorbed by the humours of the eye.