Warren De La Rue to Faraday   15 March 1850¹

Royal Institution | Friday 15th 1850

My dear Sir

I have bought up the slide Mr. Nobert sends for your acceptance. You will, with the unassisted eye, perceive only two lines one broad, the width of which is 1/2 a Paris line or the 1/24th of a Paris inch (the 12th part of the old pied du roi) divided into 500 lines - there are actually 500 lines there I can vouch for, having counted them. The narrow line contains 15 series of these wonderful lines of Noberts, getting gradually finer - the enclosed piece of paper contains a table of their values the first two columns are the numbers given by Nobert the last two are the values in relation to an English inch calculated by me. I believe they are correct but I intend calculating them again and will then print off a few tables for such of my friends as possess these slides. Until I send you one you must not rely absolutely on the English values given. I have made measurements of some of the lines and find them very nearly concordant with the assigned values by Nobert.

I remain | My dear Sir | Very Truly Yours | Warren de la Rue

Michael Faraday Esq | &c &c &c

Endorsed by Faraday: Noberts line | &c &c