[21] 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.
[22] Dr. Scoffern, Belgravia, October 1867.
[23] Mr. Barrett, Nature, Sept. 19, 1872.
[24] A good instance of his caution in drawing conclusions is contained in one of his letters to me:—
"Royal Institution of Great Britain,
"2 July, 1859.
"My dear Gladstone,
"Although I have frequently observed lights from the sea, the only thing I have learnt in relation to their relative brilliancy is that the average of a very great number of observations would be required for the attainment of a moderate approximation to truth. One has to be some miles off at sea, or else the observation is not made in the chief ray, and then one does not know the state of the atmosphere about a given lighthouse. Strong lights like that of Cape Grisnez have been invisible when they should have been strong; feeble lights by comparison have risen up in force when one might have expected them to be relatively weak; and after inquiry has not shown a state of the air at the lighthouse explaining such differences. It is probable that the cause of difference often exists at sea.
"Besides these difficulties there is that other great one of not seeing the two lights to be compared in the field of view at the same time and same distance. If the eye has to turn 90° from one to the other, I have no confidence in the comparison; and if both be in the field of sight at once, still unexpected and unexplained causes of difference occur. The two lights at the South Foreland are beautifully situated for comparison, and yet sometimes the upper did not equal the lower when it ought to have surpassed it. This I referred at the time to an upper stratum of haze; but on shore they knew nothing of the kind, nor had any such or other reason to expect particular effects.
"Ever truly yours,
"M. Faraday."
As an instance of his unwillingness to commit himself to an opinion unless he was sure about it, may be cited a letter he wrote to Sir G. B. Airy, the Astronomer Royal, who asked for his advice in regard to the material of which the national standard of length should be made:—"I do not see any reason why a pure metal should be particularly free from internal change of its particles, and on the whole should rather incline to the hard alloy than to soft copper, and yet I hardly know why. I suppose the labour would be too great to lay down the standard on different metals and substances; and yet the comparison of them might be very important hereafter, for twenty years seem to do or tell a great deal in relation to standard measures." Bronze was finally chosen.
[25] De la Rive points this out in his brief notice of Faraday immediately on receiving the news of his death:—"Je n'ai parlé que du savant, je tiens aussi à dire un mot de l'homme. Alliant à une modestie vraie, parcequ'elle provenait de l'élévation de son âme, une droiture à toute épreuve et une candeur admirable, Faraday n'aimait la science que pour elle-même. Aussi jouissait-il des succès des autres au moins autant que des siens propres; et quant à lui, s'il a accepté, avec une sincère satisfaction, les honneurs scientifiques qui lui out été prodigués à si juste titre, il a constamment refusé toutes les autres distinctions et les récompenses qu'on eût voulu lui décerner. Il s'est contenté toute sa vie de la position relativement modeste qu'il occupait à l'Institution Royale de Londres; avoir son laboratoire et strictement de quoi vivre, c'est tout ce qu'il lui fallait.—Presinge, le 29 août, 1867.—A. de la Rive."
[26] Preface to "Faraday und seine Entdeckungen."
[27] I am indebted to Sir Charles Wheatstone for the following impromptu by Herbert Mayo:—
"Around the magnet Faraday