“Aunt Jane stirred me up to sit for my picture, as she said you wished for it and were entitled to ask for it _qua_ Wrangler. I have had four sittings to Sir John Watson Gordon, and it is now far advanced; I think it is very like. It is kitcat size, to be a companion to Dyce’s picture of your mother and self, which Aunt Jane says she is to leave to you.” And now the long years of preparation were nearly over. The cunning craftsman was fitted with his tools; he could set to work to unlock the secrets of Nature; he was free to employ his genius and his knowledge on those tasks for which he felt most fitted. CHAPTER III. EARLY RESEARCHES.--PROFESSOR AT ABERDEEN. From this time on Maxwell’s life becomes a record of his writings and discoveries. It will, however, probably be clearest to separate as far as possible biographical details from a detailed account of his scientific work, leaving this for consecutive treatment in later chapters, and only alluding to it so far as may prove necessary to explain references in his letters. He continued in Cambridge till the Long Vacation of 1854, reading Mill’s “Logic.” “I am experiencing the effects of Mill,” he writes, March 25th, 1854, “but I take him slowly. I do not think him the last of his kind. I think more is wanted to bring the connexion of sensation with science to light, and to show what it is not.” He also read Berkeley on “The Theory of Vision” and “greatly admired it.” About the same time he devised an ophthalmoscope.[19] “I have made an instrument for seeing into the eye through the pupil. The difficulty is to throw the light in at that small hole and look in at the same time; but that difficulty is overcome, and I can see a large part of the back of the eye quite distinctly with the image of the candle on it. People find no inconvenience in being examined, and I have got dogs to sit quite still and keep their eyes steady. Dogs’ eyes are very beautiful behind--a copper-coloured ground, with glorious bright patches and networks of blue, yellow, and green, with blood-vessels great and small.” After the vacation he returned to Cambridge, and the letters refer to the colour-top. Thus to Miss Cay, November 24th, 1854, p. 208:-- “I have been very busy of late with various things, and am just beginning to make papers for the examination at Cheltenham, which I have to conduct about the 11th of December. I have also to make papers to polish off my pups. with. I have been spinning colours a great deal, and have got most accurate results, proving that ordinary people’s eyes are all made alike, though some are better than others, and that other people see two colours instead of three; but all those who do so agree amongst themselves. I have made a triangle of colours by which you may make out everything.