It remains to find out how to shorten down the waves--to hurry up the vibration until the light becomes visible. Nothing is wanted but quicker modes of vibration. Smaller oscillators must be used--very much smaller--oscillators not much bigger than molecules. In all probability--one may almost say certainly--ordinary light is the result of electric oscillation in the molecules or atoms of hot bodies, or sometimes of bodies not hot--as in the phenomenon of phosphorescence. The direct generation of _visible_ light by electric means, so soon as we have learnt how to attain the necessary frequency of vibration, will have most important practical consequences; and that matter is initially dealt with in a section on the Manufacture of Light, § 149, in Chapter XIV of _Modern Views of Electricity_. But here we abandon further consideration of this aspect of our great subject. FOOTNOTE: [1] See Lodge and Howard, _Philosophical Magazine_ for July, 1889. See also _Phil. Mag._, August, 1888, page 229. CHAPTER II THE INTERSTELLAR ETHER AS A CONNECTING MEDIUM So far I have given a general idea of the present condition of the wave theory of light, both from its theoretical and from its experimental sides. The waves of light are not anything mechanical or material, but are something electrical and magnetic--they are in fact electrical disturbances periodic in space and time, and travelling with a known and tremendous speed through the ether of space. Their very existence depends upon the ether, and their speed of propagation is its best known and most certain quantitative property. A statement of this kind does not even initially express a tithe of our knowledge on the subject; nor does our knowledge exhaust any large part of the region of discoverable fact; but the statement above made may be regarded as certain, although the absence of mechanics or ordinary dynamics about it removes it, or seems to remove it, from the category of the historically soundest and best worked department of Physical Science, viz. that explored by the Newtonian method. Though in truth there is every reason to suppose that we should have had Newton with us in these modern developments. There is, I believe, a general tendency to underrate the certainty of some of the convictions to which natural philosophers have gradually, in the course of their study of nature, been impelled; more especially when those convictions have reference to something intangible and occult. The existence of a continuous space-filling medium, for instance, is probably regarded by most educated people as a more or less fanciful hypothesis, a figment of the scientific imagination,--a mode of collating and welding together a certain number of observed facts, but not in any physical sense a reality, as water and air are realities.