ARTICLES 245 



aberration. To parabolise a large mirror accurately is not an 

 easy matter : the process is one of trial and error, frequent and 

 careful optical tests being required to check the progress of 

 the work. Mr. Ritchey of the Mount Wilson Observatory had 

 succeeded with the 60-inch mirror, and with the experience 

 so gained it seemed probable that the difficulties of working 

 the still larger disc would be overcome. 



(3) The design and construction of a mounting capable of 

 carrying the mirror with the necessary accuracy. The total 

 weight of the telescope would be very large, yet it must be well 

 balanced and capable of being driven in a regular manner by 

 clockwork. 



(4) Changes of focal length due to temperature variations. 

 Prof. Perrine recently stated that when figuring a 90-cm. flat 

 for use in testing a i|m. paraboloid at Cordoba Observatory, 

 it was found that when the temperature rose as much as |°C. 

 in from two to six hours the flat became sensibly convex. Unless 

 particular precautions are taken, changes of focus due to change 

 in temperature in large reflectors occur which may interfere 

 greatly with the quality of the photographs. At Mount Wilson, 

 there is the advantage that during the observing season the 

 night temperature is nearly constant after 9 p.m., and it was 

 thought that during the daytime it would be possible to main- 

 tain the mirrors at the average night temperature by means of 

 a refrigerating plant. 



As it appeared therefore that all the difficulties could be 

 overcome the 4- 5 ton disc was ordered from the French Plate 

 Glass Company of St. Gobain, France, and the erection of build- 

 ings for grinding, figuring, polishing, and testing the mirror 

 were commenced. Funds for the mounting, dome, and building 

 were still required, but it was thought that they would be 

 forthcoming. 



Careful designing of the buildings for working the mirror 

 was necessary. These consisted of a large room 34 feet square 

 by 20 feet high opening into a long testing hall, 100 feet long and 

 10 feet wide, to enable the mirror to be tested at its centre of 

 curvature or with parallel light. A constant and uniform 

 temperature is necessary for the testing, so that the room had 

 to be heated by an automatic heater. The greatest precautions 

 were taken to avoid damage from scratches during polishing : 

 double windows were provided, sealed to prevent the admission 



