Faraday to Jacob Herbert   26 December 1845

Royal Institution | 26 Decr. 1845

My Dear Sir

The answer to your last letter¹ and the extracts from the letters² of Capt Love³ is given by certain principles which have already at different times been brought to your attention⁴ and which I will as briefly as possible recapitulate. When the rays either from a naked lamp or a reflector are required to be red it is done by using a plate of red glass; the action of which is not to create red rays but to absent the other coloured rays leaving the red to pass and it is at your pleasure to use such depth of tint as shall make the whole beam of the redness you think fit. If one plate of glass placed at the front of the reflector is used and produces the proper effect, then such a plate applied against the metal of the reflector between it & the lamp would produce far too much effect for as regards the reflected rays they passing twice through it would be diminished in light as much as if two such red glasses were at the front of the reflector. If to compensate for this effect a glass of only half the tint were used, it would produce the same effect (theoretically) as the one deeper glass at the front of the reflector & in that respect the effect would be the same. But practically it would not be the same but worse in some degree on this account that it would be almost impossible to make the glass so true in figure & adaptation as that it should not by oblique refraction disturb the course of the rays far more than a flat plate of glass in front; & hence more light would be scattered & thrown out of the required direction.

In respect of the effect it would give of making the whole beam more intense & brilliant by the addition as it were of the beams direct from the lamp, I may first observe that it would be only the light equal to part of that of a naked lamp that would be added to the coloured beam & this as you are aware is a very small proportion. All the reflected light would be coloured by going twice through glass.

Next I may remind you of certain experiments which were made by order of the Deputy Master⁵ at Mr. Wilkins house in Long Acre in which part of the light was allowed to pass from the reflector as white light, to add to the brilliancy of the red ray passing through the red glass, but I think that the result of these experiment[s] was that red colour was then too much lost & the whole ray was not red enough. On principle it is better if a given amount of light is to be employed not to make one portion red to a certain degree & then mix with it the other portion as white but to make all red; only to a less degree letting in fact more white light pass through the glass in this way the two points of colour & luminosity conjoined are obtained in their best condition. I think you will upon recollection consider points 2 & 3⁶ answered by the experiments at Mr. Wilkins[.]

I have lately had occasion to make rough coloured reflectors using silvered surfaces covered with red varnish & they had a very good effect whilst the varnish was perfect but not so good as a plate of ruby glass before a clear silvered reflector. Red glass reflectors could be made & silvered but I think they would be more liable to casualties than silvered copper reflectors and not be so good in shape. A red reflecting metal like silver would indeed be a very valuable substance & is a great desideratum. I have often thought upon the matter but do not see any hopes as yet of preparing such a metal fit for lighthouse uses⁷.

I am | My dear Sir | Your Very faithful Servant | M. Faraday

J. Herbert Esq | &c &c &c