Faraday to Arthur-Auguste De La Rive   16 December 1859

Royal Institution | 16 Decr. 1859

My dear De La Rive,

Your letter¹ was a very agreeable surprize I cannot tell when I wrote to you last² but know I often think of you and that with very great pleasure since it is in the belief that you can understand more of the power of God than what can be gained by the study only of his material works[.] Yet how wonderful they are I think yours is just the mind to revel amongst them as the evidences in natural things of his eternal power & Godhead³ - and though I do not like when speaking of them in a common lecture to deal irreverently with religion by drawing it in at second hand I think it is impossible to forget who hath ordered them.

I have no doubt you have Becquerels papers from the Annales de Chimie⁴ whether he will be able (or Ruhmkorf for him) to send you some of his phosphori I do not know. I had some from him very beautiful. They are preparations of the Sulphurets of Strontia, lime and baryta - He pulverizes them & then having gummed a paper surface (a circle 8 or 9 inches in diameter) he sifts the powder over it & when all is dry knock off the loose powder & a pure phosphorescent surface is left[.] Having an Electric lamp with a lens & a divergent beam - placing the expanded hand on the surface & then throwing the beam on to it for a moment the result is beautiful for on showing it to the audience after the hand is removed there is its black form impressed on a sheet of light. These phosphori on paper or exposed to air will not keep[.] Those I had are all insensible now as I found the other day: or I would have sent some by post - but kept in tubes hermetically sealed up they remain good for years[.]

When Becquerel speaks in his letter to you of my experiments on a screen I suppose he refers to fluorescence as well as phosphorescence[.] The fluorescence is most powerfully produced by the rays at the violet end of the spectrum[.] So by throwing the spectrum obtained by passing a divergent beam from the Electric lamp through a prism placed near the lens of the lamp; cutting off the red orange yellow rays &c by a screen placed near the prism - a beam of feeble light is obtained on the large white screen well fitted to bring out the fluorescent results of Uranium glass - Quinine, [word illegible] &c[.]

I have sent you a printed report of my evening⁵ - by the regulations here it seems to require only a penny stamp but I have been so often disappointed by the uncertainty of the Post office proceeding that I have not usually sent these unimportant papers to you. I only hope you will not have more to pay[.] The report is not worth more[.] However in it you will see described a phosphoroscope which I had made for the evening & which answered exceedingly well especially with the nitrate of uranium. I think you will have no difficulty in comprehending it. The cylinder described was in a vertical position and no part of it more than 10 or 12 inches distant from the Electric focus[.] It was closely made & blackened so that no light escaped outwardly but such as was brought out by the phosphorescent or fluorescent substance[.]

I am glad to hear that you think of paying England a visit[.] I look upon you as a young man - and then again as a strong young man. I know you can go on working in a manner which astonishes me I look on & admire - I rejoice with you & Madame De la Rive and though I wish for the power of imitating I do not envy you[.] It is surely my time to rest. A new year is coming and a new period & life may they be happy to us and ours is the earnest & I desire it to be the only wish of your old friend[.]

My wife desires to join me in the sincerest thoughts of good to you and Madame de la Rive.

Ever My dear friend | Affectionately Yours | M. Faraday