“Such, we think, are the details which you required: we wish that they may prove agreeable to you.
“We beg you to accept the assurance of the high consideration with which we are, Sir,
“Your very devoted Servants,
“VINCENT CHEVALIER, ainé et fils.”
“69, Quai de l’Horloge.”
[41] The objects employed by me in looking into the defects and excellencies of these glasses, were an artificial star, and a piece of enamelled dial-plate, the phenomena presented by which, when put out of focus, incontestably warrant the judgment I have pronounced upon them, as any, true optician will admit.
[42] When an object-glass is out of adjustment, its maximum of distinctness is not in the centre of the field of view, but somewhere else, according to circumstances.
[43] The question must naturally resolve itself into this point, for achromatics for microscopes, (as they are called,) were made by Dollond, Martin, and Pollard, many years ago; the only objection to them was, that they were not effective, consequently, nominal only, and useless. I defy any man to produce an effective object-glass, which can satisfactorily be proved to have been made before 1824.
[44] See the first instruction published by Messrs. C. along with their microscopes: “Microscope Achromatique selon Euler,” &c. I apprehend there is a misdate in Messrs. C.’s letter at the end of this paper, where they state they have made the achromatics since 1824, with four objectives, which implies that they also made them before that period, but in another way.
[45] Mr. C. Tulley recommended the combination of two achromatics together from nearly the beginning of his son W.’s labours upon them, who rejected the idea, together with myself, as giving rise to too great a complication, always supposing that triple glasses must be used. I myself combined two triple achromatics together, for experiment sake, in a very early stage of our proceedings, but liked not the result, though effects were certainly produced by the composition, which could not be obtained from the best individual triple one.