Page:Astronomy for Everybody.djvu/88

Then followed a still larger instrument, thirty inches in diameter, for the Observatory of Pulkova, Russia. Next was completed the thirty-six-inch instrument of the Lick Observatory, which has done such splendid work.

After the death of Feil, the business was taken up by Mantois, who made optical glass of a purity and uniformity that no one before him had ever approached. He furnished the disks with which the Clarks figured the objective for the Yerkes telescope of the University of Chicago. This is about forty inches in diameter, and is the largest refracting telescope now in actual use for astronomical purposes.

Our readers have doubtless been interested in the great telescope of the Paris Exposition of 1900, which is yet larger than that of Chicago, being of forty-seven inches aperture. This instrument is of such immense size that it cannot be mounted and pointed at the heavens in the usual way. It is therefore fixed in a horizontal, north and south position, and the rays of the object to be observed are reflected into it by an immense plane mirror. The question whether this contrivance has been successful with so large an instrument is one that is not yet settled with astronomical precision. Nothing has yet been done with this instrument, which, it is feared, is so imperfect in make as to serve no better purpose than that of a toy.

The engineering problem of mounting a great telescope is by no means a simple one. It was one in which Mr. Clark was less successful than in the construction of his object-glasses. In the case of the later telescopes the