I may observe, that Mr. Lister has combined that marked 10, with 4, and finds the performance proportional to that of 4 and 2.
It will be remarked that Messrs. C., from an apparent ignorance of the value of aperture; and perhaps impressed with the too common and prevalent idea, that, having once obtained distinctness and achromatism in their object-glasses, every thing else might be accomplished by a condensation of artificial light, have reduced their apertures to such a degree, as to render their instrument as ineffective upon test-objects as a common compound; for when the opening of an aplanatic glass is cut off to the same diameter as a common one, it shows _nothing more_, though it will certainly exhibit objects far _more satisfactorily_. Upon the apertures of microscopic lenses their effects entirely depend, as was remarked a long time ago by the great Huygens. An achromatic glass is more valuable than another, merely on account of the larger aperture it will bear, without causing aberration, and consequent indistinctness. Those who are in possession of Messrs. C.’s microscopes should get the stop behind the object-glasses turned out, and procure others to be used ad libitum, according to what the goodness of the object-glasses will permit.
I feel myself called upon, however, to state, that, since the completion of Mr. L.’s microscope, Messrs. C. _have enlarged the_ [p252] _apertures of their object-glasses to the requisite angle_, and have moreover _arrived at the true method of adjusting them_, so that they are _now_ free from those objections which applied to Mr. Lister’s, and are in all respects unexceptionably finished.
I know not if any dispute will ever arise hereafter, as to who is to be considered as _the original maker of effective aplanatic object-glasses for microscopes_[43]. It is of very little consequence in the present instance, for it so happens that Mr. Tulley and Messrs. Chevalier have been so totally unconnected with each other, and have worked upon such totally different principles, that it must be evident, on the most superficial consideration, that both are entitled to the honour; nevertheless I apprehend it can be proved that Mr. Tulley made an effective one before the Chevaliers, having completed his in March 1824. The date affixed by Chevalier to his first instrument is 1825[44].