From Leonard Darwin   28 November 1877

Dear Father

I enclose the calculations; but see my remarks on one of them.¹ I am nearly sure that the yellow glass in that cover was originally from one of those old wooden frames that you had years ago. The glass was broken and I had it put in that cover for some experiment of yours but at that time you did not use it.

Judson’s crimson dye is the dye used for that gelatine; I believe nothing that lets light through, altogether stops the actinic effects of light, but I should be almost certain that that gelatine would stop it as much as any colour would do so. After you have done your experiments with the gelatine, I could try, very roughly, how much of the actinic rays for silver salts were stopped. If you want any more of those coloured sheets you had better let me have them made here.²

I am so glad they have had a fine day for the wedding, it would have been dismal in rain.³ I shall be home on Saturday

Your affec son | L. Darwin