genius amongst you—­not sown broadcast, believe me, it is sown thus nowhere—­but still scattered here and there.  Take all unnecessary impediments out of its way.  Keep your sympathetic eye upon the originator of knowledge.  Give him the freedom necessary for his researches, not overloading him, either with the duties of tuition or of administration, nor demanding from him so-called practical results—­above all things, avoiding that question which ignorance so often addresses to genius:  ‘What is the use of your work?’ Let him make truth his object, however unpractical for the time being it may appear.  If you cast your bread thus upon the waters, be assured it will return to you, though it be after many days.

APPENDIX.

ON THE SPECTRA OF POLARIZED LIGHT.

Mr. William Spottiswoode introduced some years ago to the members of the Royal Institution, in a very striking form, a series of experiments on the spectra of polarized light.  With his large Nicol prisms he in the first place repeated and explained the experiments of Foucault and Fizeau, and subsequently enriched the subject by very beautiful additions of his own.  I here append a portion of the abstract of his discourse:—­

’It is well known that if a plate of selenite sufficiently thin be placed between two Nicol’s prisms, or, more technically speaking, between a polarizer and analyzer, colour will be produced.  And the question proposed is, What is the nature of that colour? is it simply a pure colour of the spectrum, or is it a compound, and if so, what are its component parts?  The answer given by the wave theory is in brief this:  In its passage through the selenite plate the rays have been so separated in the direction of their vibrations and in the velocity of their transmission, that, when re-compounded by means of the analyzer, they have in some instances neutralized one another.  If this be the case, the fact ought to be visible when the beam emerging from the analyzer is dispersed by the prism; for then we have the rays of all the different colours ranged side by side, and, if any be wanting, their absence will be shown by the appearance of a dark band in their place in the spectrum.  But not only so; the spectrum ought also to give an account of the other phenomena exhibited by the selenite when the analyzer is turned round, viz. that when the angle of turning amounts to 45 deg., all trace of colour disappears; and also that when the angle amounts to 90 deg., colour reappears, not, however, the original colour, but one complementary to it.

’You see in the spectrum of the reddish light produced by the selenite a broad but dark band in the blue; when the analyzer is turned round the band becomes less and less dark, until when the angle of turning amounts to 45 deg. it has entirely disappeared.  At this stage each part of the spectrum has its own proportional intensity,