148 re;ports of investigations and projects. 



60-inch Reflector. — In April, 1907, when the 60-inch mirror was receiving 

 its finishing touches, it was found one morning that the entire surface was 

 covered with scratches. How these were produced is a mystery which has 

 not been solved. The extreme precautions taken to avoid scratches, some 

 of which were mentioned in my last report, have been rigorously observed 

 during the year. Indeed, it is difficult to see how greater care could have 

 been exercised by the opticians. It can only be supposed that some very 

 small but hard particles, not removed in the processes of washing and strain- 

 ing always employed, were present in the rouge. However caused, the 

 scratches were so serious as to require regrinding of the mirror, which had 

 to be brought back to the spherical form. In other words, the entire work, 

 except the rough grinding and the polishing of the rear surface, had to be 

 done over again. The advantages of experience are illustrated by the fact 

 that the work was entirely completed in August, with an accuracy so great 

 that no residual error exceeds one-tenth of a wave. The Institution is 

 certainly to be congratulated on the rapidity and success of this work, the 

 great difficulties of which can be appreciated only by those who are familiar 

 with the technical details involved. The final tests of the mirror were made 

 with parallel light, secured by the aid of a plane mirror 36 inches in diameter, 

 made by Mr. Ritchey for this purpose. A very fine 36-inch spherical mirror 

 was also made for the purpose of testing the plane mirror. The other work 

 of the optical shop includes the figuring of various plane and convex mirrors 

 for the 6o-inch reflector and the two plane mirrors required for the tower 

 telescope. As some of these are 12 inches thick, and some elliptical in form, 

 special apparatus had to be constructed for grinding them. 



The mounting for the 60-inch reflector has advanced satisfactorily. The 

 parts, as received from the Union Iron Works Company, were set up in 

 our erecting house and much work has been done on them. The most delicate 

 operation was the cutting of the large worm-gear, 10 feet in diameter, for 

 which special machinery had to be constructed. It may safely be said that 

 the precision of this work will leave nothing to be desired. The driving- 

 clock has been completed, as well as many other parts of the mounting, 

 which are now being finally assembled. The skeleton steel tube has arrived, 

 but the end sections of the tube, which are to carry the various convex and 

 plane mirrors required for the Newtonian and Cassegrainian arrangements 

 of the mounting, have not yet reached us. 



The completion of the steel building and dome for the 60-inch reflector 

 was greatly delayed by long-continued strikes at the Union Iron Works, but 

 most of the parts have now reached Pasadena and are being hauled to the 

 mountain. The construction force on Mount Wilson, under the superin- 

 tendence of Mr. George D. Jones, has completed the large concrete pier for 

 the mounting, the pier and concrete house for the fixed spectrograph, and 

 the concrete foundations for the steel building. They are now engaged in 



