“The Amician achromatic microscope is composed of a tube seven inches long, at the extremity of which is placed a prism, which reflects at a right angle the rays which come from the object-glass, composed (_as in our microscopes executed since 1824_) of four double object-glasses, which may be used separately, or two, three, or four at a time. The stand is a square bar, which has a rackwork, carrying a moveable stage, which, by means of adjusting screws ingeniously disposed, permits an object to traverse the field of view in every direction. This disposition gives the power of determining the real dimensions of objects submitted to observation by means of the micrometer screw, which is placed at the side, while the camera lucida affords the means of drawing their outline, and consequently of measuring the magnifying power. “The rays proceeding from the object which have passed the object-glass, and have been rendered horizontal by the prism, are received by different eye-pieces disposed after the manner of [p258] Ramsden. Their power can be varied. Each instrument carries six, five of which can be attached at pleasure either to the catadioptric or the achromatic. The deepest belonging to the reflector is a single lens of half a French line focus, and the most powerful of the achromatic is a line and a half. “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_.” FOOTNOTES: [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.