"The materials," he said, "are mixed and melted in a clay pot holding from five hundred pounds to a ton, and are constantly stirred with an iron rod until the proper combination is obtained. The heat is then slowly diminished until the glass becomes too stiff to be stirred longer. Then the mass, pot and all, is placed in the annealing furnace. Here it must remain undisturbed for a period of a month or more, when it is taken out; the pot and the outside parts of the glass are broken away to find whether a lump suitable for the required disk can be found in the interior. "If the interior were perfectly solid and homogeneous, there would be no further difficulty; the lump would be softened by heat, pressed into a flat disk, and reannealed, when the work would be complete. But in practice, the interior is always found to be crossed in every direction by veins of unequal density, which will injure the performance of the glass; and the great mechanical difficulty in the production of the disk is to cut these veins out and still leave a mass which can be pressed into a disk without any folding of the original surface." The glass for a telescope is usually composed of a double convex lens of crown glass, and a plano-concave lens of flint glass. M. Feil & Sons made and shipped the latter, which weighed three hundred and seventy-five pounds, but broke the crown glass in packing it. Then during three years they made twenty unsuccessful trials before obtaining a perfect glass. The cutting away of the clay pot and outside glass is a tedious process, requiring weeks and even months. No ordinary tools can be used. The pieces are "sawed by a wire working in sand and water.... When it is done," says Professor Newcomb, "the mass must be pressed into the shape of a disk, like a very thin grindstone, and in order to do this the lump must first be heated to the melting-point, so as to become plastic. But when Feil began to heat this large mass it flew to pieces." He took more and more time for heating, and finally succeeded. The noted firm of Alvan Clark & Sons of Cambridge, Mass., did the polishing and shaping of the lenses, a labor requiring great skill and delicacy of workmanship. The objective glass was ordered in 1880, and reached Mount Hamilton late in 1886, having cost $51,000. It weighs with its cell 638 pounds. The Clarks would not undertake any larger objective than thirty-six inches. This was six inches larger than the great glass which they had made for the Imperial Observatory at Pulkowa, near St. Petersburg in Russia.