It is perfectly achromatic with its clear aperture, and may be used without a stop on most transparent objects; requires to be cut off to 0.23, to give the necessary distinctness for opaque ones.—(When I speak of the apertures which C.’s lenses will bear, I must be understood, here and elsewhere, only with regard to the middle of the field of view, or rather that part of it where the distinctness is greatest[42], for double object-glasses give the central rays only correct, and confuse the oblique ones very much, for which reason, conjoined with the small apertures they admit of, they were abandoned by Mr. Tulley, for the triple construction, the true and regular form for the microscope.)—There is an excess of spherical aberration in convex lenses; neither are the glasses well ground, or centered, or duly adjusted. The concave of this object-glass is tarnished, and there are traces of seediness in the cement, which is, indeed, to be seen more or less in the whole of them.
10.) Focus about 0.91, clear aperture 0.23, original stop 0.09. [p250]
This object-glass is under corrected in point of colour, and wants to be made longer in the focus to be achromatic. The excess of uncorrected spherical aberration is in the convex lens; the glasses are not well ground, centered, or adjusted; the same appearance of tarnish as in 14; bears its clear aperture for the middle of the field on most transparent objects, but must be cut off to 0.2 for opaque ones.
Both of these object-glasses are ineffective upon test objects, from want of sufficient power and aperture.
4.) Focus about 0.43, clear aperture 0.23, original stop 0.09, perfectly achromatic. The uncorrected spherical aberration is in the concave; centering and grinding very fine, but in very bad adjustment; shows some transparent test-objects pretty well with its clear aperture, and, cut off to about 0.16, performs well on many opaque ones.
2.) Focus about 4.7, clear aperture 0.21, no stop, perfectly achromatic, surplus of spherical aberration in the concave as before; centering and grinding very fine; adjustment tolerable; in other respects very similar to 4. This object-glass being adjusted, does more singly on test-objects than any other, and carries an aperture of 0.16 well on opaque bodies, showing the lines on the diamond-beetle’s scales strong and well cut out.
Combination of 4 and 2—(quadruple.)
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.”