A Big Glass.

A recent visit to the workshop of the famous lens makers, Alvin Clark & Sons, Cambridgeport, furnishes some interesting facts concerning the objective for the great telescope of the Lick Observatory in California. It being impossible to get the glasses cast satisfactorily in this country, the Clarks sent to Paris after them soon after receiving the order from the trustees of the Observatory. Considerable time was spent before the first glass, the flint, was successfully cast, and this did not reach Mr. Clark until about three years ago. A very much longer delay accompanied the casting of the crown glass, which was received last October. The original castings were an inch or more greater radius than after grinding, and almost innumerable difficulties had to be overcome in order to get a perfectly clear and homogeneous piece of glass of such size.

At the present time, the flint is entirely ground and polished on one side, but on the other has yet to undergo corrections for diffraction. Whereas it is now slightly convex on this latter side it will have to be ground flat or even a trifle concave, to bring about the desired result. The crown is already ground and is being put through the process of polishing.

The grinding and the polishing are done as follows: In the first place the curves of the lens are determined by experiments with smaller models, aided, of course, by the maker's long experience. Then an iron disk, large enough to-cover the glass, is made into a concave shape exactly corresponding to the desired convexity of the lens, thus, in reality forming a species of mould. This disk, which by the way is called the "tool," is placed on the glass, and by a simple mechanical device is made to rotate upon it. When the grinding is completed by the use of this tool and grinding-powder, and the lens reduced to the proper convex form, the polishing tool is applied. This is made of a wooden disk covered with coal tar, the tar being cut up into blocks on the side which touches the lens. The surface of the tar is coated, in turn, with bees-wax, which, by mild heat and pressure upon the surface of the glass, also acquires the exact curve of the lens. With this bees-wax, rubber, and very fine rouge, the polishing is done.

When the two glasses are finally set in the brass mountings and the lens is complete, the diameter of clear glass inside the fittings will be three feet, the lens having a focal distance of 57 feet! This means that the cylinder of the telescope itself will be at least 57 feet in length, a monstrous "spy glass" indeed.

Upon being questioned as to the generally supposed superiority of such an immense lens over those now in use, Mr. Clark said that many people had a very greatly exaggerated idea of what the lens was going to do. He thought that it would not materially increase our knowledge of the heavenly bodies, and that the size and the difficulty of making were entirely out of proportion to the gain in availability.

Mr. Alvin Clark is now an old man, nearly eighty-seven years old. This magnificent piece of work seems a fitting close for a life, the industry of which has made his lenses famous throughout the world, and given one of the most important aids to modern astronomy, namely, the use of better instruments than the science ever knew before.

H.