It is a fundamental principle, that all superfluous refractions and reflections are to be avoided in the construction of optical instruments. As a radical reformer of microscopes, I can tolerate no abuses in them, or show any quarter to their abettors. Messrs. Chevalier have also undertaken the manufacture of achromatic and catadioptric microscopes, after the fashion of those made by Professor Amici, of Modena, which were so much and so justly admired by the cognoscenti of this country. It only remains for me to observe, that though two double-cemented object-glasses form the most perfect _combination_ from the fewness of their surfaces, and consequent brightness of their image, yet a fusion between the Tulleian and Eulerian constructions seems to be the _most convenient_ for general use; by this, of course, I mean a triple glass with a double one, to apply before it occasionally, à la Lister. Mr. Dollond and Messrs. C. have demonstrated that two object-glasses _may be combined with the best effect, which are both good, and work well separately_; but Mr. Tulley has constructed a double one, which, useless by itself, when applied over a triple one, (made to act singly,) corrects that excess of spherical aberration in the concave lens, (by its own excess in the convex,) which, when the aperture is large, is the eternal vice of all the best single and compound object-glasses for diverging rays which I ever saw. This is perhaps the ultimatum of improvement, though a quadruple one, on the same plan, might have the advantage in greater light and clearness, from its simplicity, and the paucity of its surfaces. The quadruple or quintuple object-glasses are those which [p255] are best adapted for the solar microscope, for they give a full-sized field of view to this instrument, good to the edges, which no single object-glass will do, as I have had occasion to remark in my paper on Mr. Tulley’s aplanatics, unless converted into a compound, by means of eye-glasses, &c. This popular and highly amusing instrument will now receive the utmost reformation and improvement of which it is capable, and become truly scientific in its construction: hitherto it has been a mere toy, but one degree removed from a magic lantern. I shall now allow Messrs. C. to say what they can for themselves, and to detail the various modifications which they have introduced into their instruments, since they executed Mr. Lister’s order, by giving a translation of a letter I have received from them on the subject, and shall conclude by expressing a hope that no national or illiberal feeling has entered into the composition of this critique, and that I have used my oil, vinegar, and pepper in correct proportions. “_Paris, Oct. 15_. “Sir,