P. 165.—I cannot answer this question. Is it possible that he was thinking of St. Augustine? In the Confessions, i. 25., we kind the expression vinum erroris.
P. 177.—No doubt Bacon had read the treatise of Sallust quoted, but my impression is that he thought the proverb had grown out of the line in Plautus.
P. 180.—I have searched again for "alimenta socordiæ," as it is quoted in the Colours of Good and Evil, but cannot fix upon any passage from which I can say it was taken, though there are many which might have suggested it. One at p. 19. of the Advancement, which I missed at first, I have since met with. It is from the Cherson., p. 106.
Thomas Markby.
PHOTOGRAPHIC CORRESPONDENCE.
Test for a good Lens.—The generality of purchasers of photographic lenses can content themselves with merely the following rules when they buy. It ought to be achromatic, i. e. consisting of the usual two pieces of crown and flint glass, that its curves are the most recommended, and that it is free from bubbles: to ascertain the latter, hold the lens between the finger and thumb of the right hand, much as an egg-merchant examines an egg before a strong gas flame, and a little to the right of it; this reveals every bubble, however small, and another kind of texture like minute gossamer threads. If these are too abundant, it should not be chosen; although the best lenses are never altogether free from these defects, it is on the whole better to have one or two good-sized bubbles than any density of texture; because it follows, that every inequality will refract pencils of light out of the direction they ought to go; and as bubbles do the same thing, but as they do not refract away so much light, they are not of much consequence.
I believe if a lens is made as thin as it safely can be, it will be quicker than a thicker one. I have two precisely the same focus, and one thinner than the other; the thinner is much the quicker of the two. An apparently indifferent lens should be tried with several kinds of apertures, till it will take sharp pictures; but if no size of aperture can make it, or a small aperture takes a very long time, it is a bad lens. M. Claudet, whose long experience in the art has given him the requisite judgment, changes the diameter of his lenses often during the day; and tries occasionally, in his excellent plan, the places of the chemical focus: by this his time is always nearly the same, and the results steady. As he is always free in communicating his knowledge, he will, I think, always explain his method when he is applied to. The inexperienced photographer is often too prone to blame his lens when the failure proceeds more from the above causes. The variation of the chemical focus during a day's work is often the cause of disappointment: though it does not affect the landscape so much as the portrait operator.
If any one has a lens, the chemical and visual focus being different, his only remedy is M. Claudet's method. And this method will also prove better than any other way at present known of ascertaining whether a lens will take a sharp picture or not. If, however, any plan could be devised for making the solar spectrum visible upon a sheet of paper inside the camera, it would reduce the question of taking sharp pictures at once into a matter of certainty.