The great merit of lead-glass lies in its absolute transparency and brilliancy, combined with a certain darkness in the shadows. This brilliancy and fire, it is well to point out, are only indirectly dependent upon the refractive power exercised by the glass upon the rays of light that pass through it; in this respect lead-glass differs little from rock crystal or from the Venetian _cristallo_. But one quality it has which distinguishes it from all other kinds of glass as well as from nearly all transparent natural stones, the diamond, of course, excepted. This is the power possessed by it of _dispersing_ the rays of white light: the elements of which this light is composed in passing through lead-glass are bent aside in _different degrees_, so that the issuing ray is broken up into its component colours. This it is that gives fire, but this fire is only fully brought out by means of facetted or angular surfaces. On this point—the distinction between refraction and dispersion—a good deal of confusion exists. The following table, which I borrow from a little book on gems by Professor Church, may help to clear up this point:— Refractive Index. Comparative Dispersing Power. Diamond, 2·75 44 Flint-glass, 1·57 36 Rock-crystal, 1·55 14 Plate and crown glass, 1·52 15 We here see that lead-glass or flint-glass has little greater refractive power on light than rock crystal or the ordinary plate and crown glass of commerce which belongs to the same family as the _cristallo_ of the Venetians. In dispersive power, on the other hand, it stands apart from both these substances and rivals the diamond in scattering the component rays of white light. CHAPTER XX ENGLISH GLASS OF THE EIGHTEENTH CENTURY We may probably regard the reign of William III. as the turning-point in the history of our English glass as in so many other of our minor arts. It is to that period that one must assign the first beginnings of our modern industrial life,[252] and it is in the Dutch influence, at that time so dominant, that the principal stimulus is to be found. Of the window and mirror glass of the period a most interesting series is preserved at Hampton Court. Many of the panes of the windows facing the garden façades of the palace are strongly tinged with purple, a result of the process by which the colourless protoxide of manganese is reconverted into the purple bin-oxide under the influence of sunlight. Placed between the windows in William III.’s state bedroom are some curious mirrors with frames ornamented with _appliqué_ plates of deep blue glass carved into patterns and monograms. Observe, too, a charming mirror of the same period over the fireplace in this room.