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.

The Siderostat — A Wonderful Instrument to Magnify, View the Moon

1895

Moon at Close Range

A Wonderful Instrument Will Limn Her Figure

Magnified for Us — And at Last We Shall See If She Has Any Queer People on Her — A Frenchman's Hope May Be Realized

Not only to-day but in all ages men have been anxiously endeavoring to solve the mystery of the moon. The ancients made her their goddess, and entered upon no new undertakings without first asking her advice and consulting the probabilities of the influence with which even to-day many superstitious persons endow her.

The lens has long been regarded as the means by which we will be enabled, if at all, to study the moon's mysteries. The only question has been how to make a lens large enough. It seems to have been recognized that there is a limit to what may be called the carrying power of a lens. Alvin Olark, of Cambridge, is the man who makes big lenses, and he has about reached his limit.

Two of these record-breaking lenses are in the great Chicago telescope, and two more are about to be set in the recently mounted telescope in the Paris Observatory. A French scientist, M. Deloncle, proposes to use these lenses for a novel purpose. The plan is said to be more than three-fourths realized. It will require glasses of over forty-nine inches in diameter. A famous French lens-maker is at work on these glasses for an instrument which M. Deloncle calls the siderostat.

With this M. Deloncle hopes to interview the moon and to establish once for all whether that interesting and much-discussed planet is inhabited. It will tell us whether the moon has cities, monuments and larger buildings and also explain its signals to us, if it has any.

Says a French writer of it: "The invention is an ingenious one and demands for its perfection a man, convinced he is right, having, so to speak, plenty of 'pluck.' It is the natural outcome of all that has gone before, from Metius to Mantois; but it belongs especially to the particular genius of its author, because no one before him, not even the most learned astronomers, had dared to imagine so strange and yet so useful an instrument for stellar observation. It is the cannon of 103 tons, the Eiffel Tower, the Great Eastern of optics. The Siderostat surpasses anything of its kind. It does not even bear any resemblance to existing telescopes.

"The spectators will stand and it will lie, pointing its long horizontal tunnel at a mirror fourteen inches in thickness and weighing 13,000 pounds. The image of the satellite will be reflected, and coming out of the ocular will be projected on a screen upon which two, three, five hundred spectators may see the moon. A magnifying glass will enlarge the image to enormous proportions and a very delicate mechanical contrivance will set the mirror in motion, so that it will follow the apparent movements of the planet. The scheme will be gigantic; it will truly be 'Twentieth Century!' The question arises among the doubtful: Will it be practical? After all, negat alter!

"Unfortunately Mme. Moon, the indispensable prima donna of this lunar theater, often disappoints us. In the first place, there will be many evenings when, following in the footsteps of her capricious human sisters, she will decline to come forth from the shadows of her cloudy boudoir. Photography may be brought into requisition, and, like the understudies on the real stage, take the role of the sulking prima donna. In which case the siderostat will be vulgarized to the mean part of a huge magic lantern, which misfortune, for the sake of science, it is to be hoped will never transpire."

—The Perry Bulletin, Perry, IA, Oct. 16, 1895, p. 2.