My dear Sir

I thank you most sincerely for your glass-specimens, (which will be of the greatest use to me) & for your very kind letter.¹ The latter came quite in time; for I have only finished a few days ago, your very interesting volume (2^d. Edit) on Light.—² The chapter on the relation of light & actinism towards plants is most suggestive,³ & I had determined on trying the 3 viz blue, red & yellow, which you have sent me. But I want extremely to beg one other favour, viz where can I get such glasses.? I have not the least idea where to try?

Do you know with what the Blue & Red (& yellow) (which I think will be the most valuable to me) are coloured; as this, perhaps, would be an aid in getting the right sorts; for I infer from what you say, that the mere colour is not always a safe guide in regard to the power of the glass in intercepting the radiant powers.

After I have been to some glass place, would you permit me to show you any specimens, the manufacturer may have by them? I shall be in London on 7^th, 8^th & 9^th of August, & with your permission will call & show you any glass which I could meet with. If you will send me a line about glass manufacturers you will complete your kindness, for which I am really very much obliged to you.

Pray believe me | My dear Sir | Your’s sincerely | Charles Darwin