The question why substances transmit some rays and quench others, brings us into the domain of molecular physics. Of all branches of physical science this is perhaps the most fascinating and the most speculative, yet it is one which is being built up on the solid foundations of experiment and mathematics, till it has attained an importance which the questions depending on it fully warrants. We have to picture to ourselves, in the case in point, molecules, and the atoms composing them, of a size which no microscope can bring to view, vibrating in certain definite periods which are similar to the periods of oscillation of the waves of light. At page 26 we have given the lengths of some of the waves which give the sensation of coloured light. Now as light, of whatever colour it may be, is practically transmitted with the same velocity through air which has the same density throughout, it follows that the number of vibrations per second of each ray can be obtained by dividing the velocity of light in any medium by the wave-length. The following table gives roughly the number of vibrations per second of the ether giving rise to the colours fixed by the dark solar lines. +-----------------------+-----------------+ | Name of Line. | Millions of | | | Millions of | | | Vibrations | | | per Second. | +-----------------------+-----------------+ | A in the Red | 395 | | B " " | 437 | | C " " | 458 | | D " Orange | 510 | | E " Green | 570 | | F " Blue | 618 | | G " Violet | 697 | | H " Ultra-Violet | 757 | +-----------------------+-----------------+