Telescopic Lenses Now Complete

Massachusetts, 1896

The great lenses, forty inches in clear aperture, for the Yerkes telescope, are now complete in the workshop of Alvan G. Clark at Cambridgeport, Mass.

An observatory, to be under the control of Chicago University, has been equipped for the reception of the great telescope, and it will soon be in use. The tube, which is of steel and sixty-three feet long, was made in time for exhibition at Chicago during the Columbian Exposition, and is said to be equal to all the demands for strength, rigidity and easy movement.

The two lenses composing the objective are of the simplest form. One is of crown and the other of flint glass, each being forty and a half inches in diameter. An inch and a half is cut off in mounting, giving a clear aperture of forty inches. The crown lens is double convex, three-quarters of an inch thick at the edge and two and three-quarters of an inch thick at the center. Being well supported about the circumference, this thickness gives sufficient rigidity, although the weight is nearly two hundred pounds. The flint or negative lens is plano-concave, two inches thick on the edge and about an inch and a quarter in the center, weighing 3000 pounds. These lenses have been tested for months by Mr. Clark, and local imperfections have been corrected in the most careful manner.

The production of the rough disks of glass was a labor of great difficulty, and the final success of the makers in Paris marks a great advance in the manufacture of optical glass. One of the disks was completed and delivered at Cambridgeport long before the other was perfected, and it was necessary to await the production of both before the labor of grinding and polishing could be undertaken.

It would not be surprising if a telescope of forty-eight inches aperture were constructed within the next ten years and perhaps earlier. Of course, the possible sagging of the glass from its own weight may become perceptible in a glass of fifty inches. But the perfection of the forty-inch glass shows that the limit has not been reached, and no one can tell whether properly proportioned glass of forty-eight inches will sag or not until the experiment be tried. — Rochester Democrat and Chronicle.