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Newton surmounted this difficulty by laying a double convex lens, the radius of the curvature of each side of which was fifty feet, upon the flat surface of a plano-convex object-glass, and in the way he obtained a plate of air, or of space, varying from the thinnest possible edge at the centre of the object-glass where it touched the plane surface to a considerable thickness at the circumference of the lens.
In a poem upon stones attributed to Orpheus, it is said that the sacred fire was also lighted by a bit of crystal which concentrated the rays of the sun upon the material to be inflamed. This process must have been the one that was most usually employed before fire became common. In fact, a plano-convex crystal lens has been found among the ruins of Nineveh.
Bricks made this way were called plano-convex because five faces were flat and the sixth convex; each bore the imprint of a thumb on the convex face, formed as the brick was ejected from the mould. Similar bricks were used in the building of the Sumerian city of Ur several thousands of years ago.
We wandered along the excavated streets that were bordered by low walls of sun baked plano-convex bricks and marvelled at the state of preservation. Although the Romans are generally credited with its introduction there was, dating back to a much earlier period, a semi-circular arch still existing. In places one could see, outlined by bricks, the formation of the rooms of houses.
The former generally goes under the name of its inventor, Ramsden, and the latter is named after great Dutch astronomer, Huygens. The Huygens eyepiece consists of two plano-convex lenses whose focal lengths are in the ratio of three to one. The smaller lens is placed next to the eye.
In the Ramsden eyepiece two plano-convex lenses are also used, but they are of equal focal length, are placed at a distance apart equal to two thirds of the focal length of either, and have their convex sides facing one another.
In still others erosion has stripped off the covering strata and has laid bare the core to its base; and its shape is here seen to be that of a plano-convex lens or a baker's bun, its flat base resting on the undisturbed bedded rocks beneath. The nucleus of igneous rock is four miles in diameter and more than a mile in depth.
John Stuart, for his enterprise in securing for this country a man of such profound acquirements. The subject of distortion was next treated of, and the manner in which the idea of a non distorting doublet could be evolved from a single bi-convex lens by division into two plano-convex lenses with a central diaphragm was shown.
Fragments of shell and mother-of-pearl, often with incised designs, are very characteristic of the earliest period. One of the most useful criteria of age is: Bricks. The form of the brick is a very good guide to date. The Babylonians used both kiln-baked and crude bricks. The oldest type, whether baked or crude, is plano-convex in form, and uninscribed. The mortar is bitumen.
William Hyde Wollaston, one of the greatest and most versatile, and, since the death of Cavendish, by far the most eccentric of English natural philosophers. This was the suggestion to use two plano-convex lenses, placed at a prescribed distance apart, in lieu of the single double-convex lens generally used.