Note 21 — SECTION IV. HIS METHOD OF WORKING.

Since writing the above I have come across a letter written by Faraday in answer to one by Captain Welier as far back as 13th Sept. 1839, in which he pointed out the mal-adjustment of the dioptric apparatus at Orfordness. In July of the following year he made lengthy suggestions to the Trinity House, in which he proposed using a flat white circle or square, half an inch across, on a piece of black paper or card, as a "focal object." This was to be looked at from outside, in order to test the regularity of the glass apparatus. He also suggested observations on the divergence by looking at this white circle at a distance of twenty feet at most. Another plan he proposed was that of lighting the lamp and putting up a white screen outside. These methods of examining he carried out very shortly afterwards at Blackwall, on French and English refractors, but it seems never to have occurred to him to place his eye in the focus, or in any other manner to observe the course of the rays from inside the apparatus.