A Method of Testing Optical Glass

Abstract

It frequently happens that a large object-glass or prism, after going through the long and expensive process of grinding, polishing, and figuring, must be rejected because of internal defects in the glass. Not long ago the Solar Observatory received from a celebrated firm in Germany a large prism which was so defective that the D1 line seen through the upper part of the prism fell upon the D2 line observed through the lower part. The figure of the surfaces was excellent, and the whole difficulty lay within the glass. Subsequent attempts to obtain large blocks of glass for prisms have been equally unsuccessful, and as the same difficulty has been experienced with several objectives (including one of 12 inches aperture for the 150- foot tower telescope), the importance of avoiding the loss of time and the expense thus involved is apparent. It would seem probable that the following simple process of testing glass blocks or disks, without giving them a perfect optical figure, must have suggested itself to others. But perhaps it has serious defects which a trial, soon to be made here, will reveal, for I understand that all opticians now complete the figuring of objectives and prisms before they can discover inequalities of structure less obvious than those shown by the polariscope.