Faraday to Jacob Herbert   17 March 1843

Royal Institution | 17 March 1843

Dear Sir

Captain Drew having called my attention to an investigation he had set on foot in which red & yellow glass was combined in various ways with the view of rendering the power of the red lights used by the Trinity house more effectual & luminous I was led to make a few enquiries & experiments on the subject and by Direction of the Deputy Master¹ beg to offer this brief report of the results[.]

In order to have the power of superposing different tints of red & yellow I desired Mr. Wilkins to procure four red & 4 yellow glasses[.] Red No. 1. was to be of the intensity used in the Trinity lights. Red No. 2 was to have only half the colour or tint of No. 1. Red No. 3 was to be a fourth of No. 1 in intensity & Red No. 4 an eighth of No. 1. The same was to be done with the yellows Nos 1, 2, 3, & 4. This being accomplished it appeared that the Red No. 1 was not a true red tint i.e red in all its degree for R No. 2 was only orange or brown red. R No. 3 yellow brown & R No. 4 was a yellow of less intensity than yellow No. 1.

In fact therefore the Red No. 1 was only yellow heaped upon yellow until nearly all the light was stopped what passed by red. This was also fully shown by combining the yellow glasses for putting them one behind another the effect of the whole was to transmit a red light like that of R No. 1.

These results led me to enquire after a true red or ruby glass which I knew to be in the possession of Messrs. Chater & Hayward² of St. Dunstans hill and I obtained a sample. I found the light which passed through this glass was of a far finer colour & much more abundant than that passing through R. No. 1 or the ordinary red Glass. The test of colour was not uniform being deeper near the edges than at the middle but both at the middle & edges the glass was of a beautiful red colour having nothing of the yellow or brown shewn by the other glass when made less deep in tint[.]

I made some experiments on the quantity of light obtained from a lamp when burning steadily either without any glass before it or with one or other of the glasses referred to & though the diffusion of colour causes a difficulty in the appreciation yet the results I give being the average of several experiments made by two persons will illustrate the loss of light by red glass & the enormous loss by the R No. 1 or that given to me as ordinary English red glass[.]

Though these results must not be considered as exact in relation to the whole light stopped yet they are not far wrong as regards the relative powers of the different red glasses & from them it appears that Chaters deepest glass lets nearly 4 times as much fine red light go through as R No. 1 & the paler part allows 6 times as much light to pass. If we compare R No. 2 which is only a brown red colour to the Ruby glass, it stops more light than either the deeper or brighter part & allows little more than half as much to pass as the paler ruby glass.

All these glasses are now at the Trinity house & are easily compared as to their general effect by putting them upright on a sheet of white paper & either looking at the paper through them or letting the light pass through them on to the paper & observing the degree of colour & illumination of the paper so illuminated or by putting a piece of thin white paper over the glasses & holding both at once between the eye & a window or sky light or lamp.

According to my instructions I have enquired at Messrs Chaters & Hayward about the supply & price of the glass. It is imported by them from France & is in Sheets about 24 inches square more or less[.] Its price in the sheet uncut is 7/- per square foot for cash. If cut into smaller square or rectangular shapes it is 8/- per square foot. Pieces may be bent into the form of some cylinders according to a pattern given at the additional charge of 1/- per square foot no count being taken of the number of pieces but of the amount of glass which they altogether make up³.

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

Jacob Herbert Esq | Secretary | &c &c &c &c