ARTICLES 249 



and 150 gallons of distilled water were used. The mirror was 

 finished during 1916. 



Meanwhile progress had been made with the construction of 

 the mounting and the dome. The provision for these was made 

 possible in 191 2 by a gift from Mr. Carnegie to the Carnegie 

 Institute of Washington. Mr. Hooker, the donor of the mirror, 

 had died in 1916, and so did not live to see the completion of 

 the project which he had originated. It was decided that 

 the mirror should be used in three ways, giving different equiva- 

 lent focal lengths, viz. (1) in the principal focus, to avoid loss of 

 light, possible distortion of the field, and exaggerated change of 

 focal length due to changes in the figure or position of the 

 auxiliary mirror : in this method the light is deflected to a focus 

 at the side of the tube near its upper end by a small optically- 

 worked plane mirror of elliptical shape, placed just inside the 

 focus of the paraboloid and at an angle of 45 ° to the axis ; (2) 

 in the Cassegrain form, the light being then reflected back from 

 the upper end of the tube by a small convex mirror, perpen- 

 dicular to the axis, and brought to a focus near the lower end : 

 equivalent focal lengths up to 300 feet can thus be obtained ; 

 (3) in the coude form, in which the cone of rays from a convex 

 mirror at the upper end of the tube strikes a diagonal plane 

 mirror placed at the intersection of the polar and declination 

 axes and reflects the light in a constant direction, viz. towards 

 the south pole of the heavens. This method has the great 

 advantage that the focus remains in a constant position as the 

 telescope is moved so that it is possible to use it for high-dis- 

 persion photography of stellar spectra with a long- focus spectro- 

 graph without the necessity of the spectrograph being attached 

 to the telescope itself. 



The dome and mounting were designed by Messrs. D. H. 

 Burnham & Co. of Chicago, to particulars provided ,by Prof. 

 Hale, and their construction was carried out by the Fore River 

 Shipbuilding Works. The total mounting weighs 100 tons ; 

 the telescope tube itself is a skeleton tube, 40 feet long, built 

 up in sections. It is well shown in Plate No. II., which repre- 

 sents the telescope as it was in October 191 7, being then com- 

 plete except for the upper mirror. The type of mounting is a 

 modification of that designed by Prof. Ritchey for the 5-foot 

 reflecting telescope of the Yerkes Observatory. The tube is 

 supported by two massive trunnions fixed to a rectangular frame- 



