Page:Astronomy for Everybody.djvu/84
rarest kind, is, as we have already intimated, that of constructing the object-glass. The slightest deviation from the proper form—a defect consisting in some part of the object-glass being too thin by a hundred thousandth part of an inch—would spoil the image.
The skill of the optician who figures the glass, that is to say, who polishes it into the proper shape, is by no means all that is required. The making of large disks of glass of the necessary uniformity and purity is a practical problem of equal difficulty. Any deviation from perfect uniformity in the glass will be as injurious to its performance as a defect in its figure.[1]
A century ago it was found especially difficult to make flint glass of the necessary uniformity. This substance contains a considerable amount of lead, which, during the process of melting the glass, would sink toward the bottom of the pot, thus making the bottom portion of greater refracting power than the upper portion. The result was that, at that time, a telescope of four or five inches aperture was considered of great size. Quite early in the century, Guinand, a Swiss, found a process by which larger disks of flint glass could be made. He professed to have some secret process of doing this, but there is some reason to believe that his secret consisted only in the constant and vigorous stirring of the melted glass
- ↑ It is frequently proposed by persons not acquainted with the delicate points of the problem to make a telescope of large size by putting together different pieces of glass, each of the proper shape, to form a lens. The idea, ingenious though it looks, is thoroughly impracticable, for the simple reason that it is impossible to make two pieces of glass of exactly the same refracting power.