It is explained in Lord Rayleigh's article on Optics in the Encyclopaedia Britannica that a very minute change in the form of the curvature of the surface of a lens will make a large difference in the spherical aberration. This is to be expected, seeing that spherical aberration is a phenomenon of a differential sort, i.e. a measure of the difference between the curvature actually attained, and the theoretical curvature at each point of the lens, for given positions of point and image. Sir H. Grubb gives an illustration of the minuteness of the abrasion required in passing from a curve of one sort to a curve of another, say from a spherical to a parabolic curve, consequently the process of figuring by the slow action of a polishing tool becomes quite intelligible. In making a large mirror or lens all the processes hitherto described under grinding and polishing, etc, have to be gone through and in the manner described, and when all this is accomplished the final process of correcting to test commences. This process is called figuring. Sec. 67. Of the actual operation of this process I have no personal knowledge, and the following brief notes are drawn from the article by Sir H. Grubb, from my assistant's (Mr. Cook) experience, and from a small work On the Adjustment and Testing of Telescopic Objectives, by T. Cook and Sons, Buckingham Works, York (printed by Ben Johnson and Co, Micklegate, York). This work has excellent photographs of the interference rings of star images corresponding to various defects. It must be understood that the following is a mere sketch. The art will probably hardly ever be required in laboratory practice, and those who wish to construct large telescopes should not be above looking up the references. The process is naturally divided for treatment into two parts. (1) The detection of errors, and the cause of these errors. (2) The application of a remedy. (1) A lens, being mounted with its final adjustments, is turned on to a star, which must not be too bright, and should be fairly overhead. The following appearances may be noted:- A. In focus, the star appears as a small disc with one or two rings round it; inside and outside of the focus the rings increase in number, are round, concentric with the disc, and the bright and dark rings are apparently equally wide. The appearance inside the focus exactly resembles that outside when allowance is made for chromatic effects. Conclusion: objective good, and correctly mounted. B. The rings round the star in focus are not circular, nor is the star at the centre of the system. In bad cases the fringes are seen at one side only. Effects exaggerated outside and inside the focus. Conclusion: the lens is astigmatic, or the objective is not adjusted to be co-axial with the eyepiece.