Large Object Glasses and Mirrors
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Large Object Glasses and Mirrors book
This chapter deals with the working of object glasses and mirrors from 412 in. diameter upwards. When a lens or mirror is more than 312 in. in diameter, it is most successful in any case to work it as a single surface. The development of the silver-on-glass mirror, by avoiding the difficulties inherent in specula, began to bring the reflecting telescope back into favour in the seventies and eighties. Dr. Henry Draper's work was directed towards making mirrors of up to 1512 in. diameter for astronomical telescopes. The grinding and polishing machines were similar in principle to Dr. Draper's, but were more elaborate. The polishing rouge which Geo. W. Ritchey used was of the quality which is used in large quantities commercially in polishing plate-glass. This grade of rouge is not expensive but contains hard, sharp particles which may cause scratches. It must therefore be thoroughly washed; particulars of the process are given in the paper.