I have laid your letter of the 18th inst. [instant] before the Secretaries, and they instruct me to inform you that there are a large number of memoirs upon Spectroscopy dealing with the "lengths of heat and light waves", but the information you require would probably be more easily obtained from Everett's "C.G.S System of Units" (published by Macmillan), or some similar text-book.
Asst. Sec. R.S. [Assistant Secretary, Royal Society]'.