I am happy to be able to speak in terms of almost unqualified approbation of this composition. It, of course, surpasses the performance of any single triple-glass, _on those test-objects which require extravagant angles of aperture_. The field also is good all over; or at least would be, if the glasses were in adjustment, which is the only drawback upon it. The focus of the combination is only 0.26, yet it performs admirably on transparent test-objects with its naked aperture of 0.23, and is very fine on opaque ones with 0.16, and doubtless would carry 0.2, if the adjustment was duly carried into effect. Messrs. C. have, I think, most assuredly here hit upon one of the very best _compositions_ for the object-glass of a microscope; all the imperfections of double object-glasses, _taken singly_, are here done away, while their thinness and agglutination into one mass allows of their combining together almost as if they were simple plano-convex lenses, leaving moreover abundance of space for the illumination of opaque bodies. [p251] I must here state, that Messrs. C.’s object-glasses are all stuck together, I believe, with fused gum-mastic, or, perhaps, with very thick mastic varnish. This practice seems, in theory, to be bad, most especially if the curves united together are not of the same radius; nevertheless, practically speaking, the process of soldering seems to me to do more good, by the obliteration of two surfaces, and by keeping the glasses immovably adjusted, than harm in any other way. I cannot, in fact, discover any very sensible difference in the optical performance of these small achromatics, whether stuck together or not. I _fancy_ that they have a little more light and clearness when cemented, (as they certainly should have,) but cannot be very positive. I hold it as a maxim in practical optics, as in our common law, “de rebus non _apparentibus_ et non existentibus eadem est ratio.” I may observe, that Mr. Lister has combined that marked 10, with 4, and finds the performance proportional to that of 4 and 2.