It requires moreover a stretch of complaisance, not to be expected on this side the Channel, to be enabled to admit that the best double object-glass is (_taken singly_) effective; or that, in consequence, Chevalier made an _effective one until he had enlarged his apertures, and combined two together_[45], which combination is not to be met with in his primitive instruments. Mr. Lister, (to whom the public is mainly indebted for the present eclaircissement concerning Chevalier’s instrument,) has, by a peculiar method of his own discovery, measured the [p253] curves, thicknesses, and diameter, &c. of that marked 14, which I here give (unfortunately one of the least effective of the set,) however, in all probability, the rest are constructed on the same principle; the annexed drawing by Mr. L. will sufficiently explain itself. Nothing can surpass the beautiful simplicity of Chevalier’s, or rather Euler’s, curves, which, it will be observed, are all alike[46]. The production of _deep_ achromatics must ever be a task of some difficulty, even to those who thoroughly understand their humours and punctilios; and unscientific artists will, I think, be much more likely to succeed on the French plan, than the English one. Two double object-glasses, by themselves, are very poor things; but, when combined, perform admirably, and will, I believe, (if the three curves are of equal radii,) be far more easily executed than one of the triple Tulleian construction. The dense flint-glass of Guenard, or Frauenhofer, however, will be an indispensable requisite, from the nature of the curvatures. A triple Tulleian object-glass, and a thin double one of Chevalier, (a composition first conceived and adopted by Mr. Lister,) form an excellent combination, and give a very vivid light, without softness, dulness, or nebulosity. This, I think, is the extreme number of glasses which ought to be tolerated. Let it never be forgotten, that _a really good triple glass will bear an aperture quite sufficient for ninety-nine objects out of a hundred_. I myself denounce the practice of combining glasses together, _in all those cases where they are capable of doing their work alone_. I shall always consider it as a clumsy, bungling, and unworkmanlike method of obtaining a short focus, combined [p254] with a large angle of aperture. If a man aims at perfection, and wishes to distinguish himself in this branch of optics, let it be done by working perfect triple glasses of 0.2 and 0.3 inch focus, with 0.1 and 0.15 of perfect aperture, like those of Mr. W. Tulley and Mr. Dollond, or deeper still, if he is able; and it is with the most cordial satisfaction that I am enabled to inform my readers, that Messrs. Chevalier (duly appreciating the regular triple construction as the true form for the microscope) have applied themselves diligently to the manufacture of this species of objective, from which they have already had excellent results.