་་ Tourbelaine consists of the same rock as Mount St. Michel: I regret that the state of the tide did not allow me a personal inspection. The coast-line to the north of, and in the vicinity of Granville, presents an interesting study of one mode of the natural and gradual conversion of vertical cliff into low and sloping hills. From A to B, the cliffs are generally precipitous enough, partly from having been scarped under the fortification, and partly naturally at B the sand begins to form, not only an ordinary beach, but also an accumulation, as a glacis, 6, which thus immediately depriving the sea of access to the foot of the cliffs, (from which it not only tears out fragments, but carries them off,) and also forms a hollow, 7, which, sloping northward, forms an irregular, but on the whole, connected drain, e, of ponds, which (as in the Jersey Quenvais) entirely prevents encroachments of the same sand, that has already proved an efficient barrier to those of the sea. Returning to B, and following the course of the hills, the now undisturbed ecroulements from the top of the cliffs, successively and gradually present themselves as the ramps 1 213 41 &c. until at about two miles from Granville the 1' 23' 4' rock entirely disappears, under such a slope as 5 5' 5" covered with grass and a fine soil. C. The shaded parts always slightly rounded, as if somewhat water worn neither do they by any means bear the same aspect, although sufficiently so to conclude that they had a common origin. Care must be taken, in examining the above Figure (D), not to mistake the lines by which the Green is distinguished from the Red for cleavage lines. At a, the rock assumes the appearance of fig. B; the remainder well exemplified by C. a, has precisely the look of having been shattered by percussion, but the fragments scarcely dislocated. d, Sienite entering the Argillaceous Schist ((b) by the vein a' a' a'. On Achromatic Telescopes. HAVING perused, in the last number of the Quarterly Journal, Signor Santini's very scientific discussion on a new construction of the achromatic telescope, from which it would appear that the spherical aberration cannot always be destroyed by the separation of the lenses of the correcting glass, I beg leave to observe that this arises from the data upon which his calculations proceed being somewhat different from those which I adopted. Signor Santini supposes that the foci of the two lenses of the correcting glass are exactly equal for mean rays. This can only take place when the thickness of these lenses is evanescent : when they are of a sensible thickness, the focus of the concave must be shorter than that of the convex, otherwise their foci cannot coincide, and the combination cannot act as a plane, but as a convex lens; and it is expressly stated in the paper which was read before the Astronomical Society, that to make allowance for the thickness of the lenses, I assumed the focus of the concave lens a quarter of an inch shorter than that of the convex. Our calculations, therefore, proceeding on the different suppositions of 9 + v=0, and 9 + v = 0.25 inch, have led to different results. It is not necessary to limit the construction to the condition of the correcting lens acting as a plane; it is rather advantageous to render it slightly concave by making the difference of the foci of its component lenses somewhat greater than is necessary to compensate for the thickness of these lenses. This enables us, with the smallest possible curvatures, to create an excess of concave spherical aberrations, which can always be removed by reducing the aperture of the concave lens, and this is done by separating the two lenses. I consider myself indebted to Signor Santini for the interest he has taken in the construction; and should be happy if so eminent a mathematician were to repeat his calculations, assigning a shorter focus to the concave lens; when, I hope, our results would no longer disagree. If the destruction of the aberrations could not be completed OCT.-DEC. 1829. 2 C |