MOUNT WILSON SOLAR OBSERVATORY. 275 



installed. A special form of Stark tube, permitting the easy substitu- 

 tion of anodes of different elements and the use of an auxiliary current 

 for heating the anode, is under construction in our shop. 



The Koch microphotometer has been completed and is in constant 

 use. The permanent plate-holder, large enough to accommodate 

 plates 8 by 40 inches, has been built, a new cylindrical lens and mount- 

 ing have been provided, and a housing fitted over the mirrors in the 

 projection system. A new precision electric contact, with fittings for 

 automatically printing fiducial fines upon the plate, has been installed 

 and tested. The battery has been tested after about a year's opera- 

 tion and is in excellent condition. The instrument has been used for 

 investigations of the Stark effect on the sun, the general magnetic field 

 of the sun, solar rotation, and the examination of many laboratory plates. 



The 100-kw. transformer mentioned in the last report was installed 

 after some delay by the Southern California Edison Company in the 

 spring of this year and tested by them for long-continued service at full 

 load. This has rendered possible the resumption of experiments with 

 the ''tube-arc" under improved conditions and furnishes greater power 

 and flexibility for work with the electric furnace. 



A vacuum-arc chamber has been constructed and used for aconsider- 

 able time. It is provided with a water-cooling system and a pressure 

 gage and may be operated indefinitely without difficulty. Pressures as 

 low as 0.5 cm. of mercury are easily attained with it. 



The apparatus for cathodic deposition of metals has been rebuilt 

 to carry much larger currents. Films of silver, gold, platinum, and 

 stellite of great uniformity have been produced by this means. 



An improved form of hand-regulated arc lamp has been constructed. 

 It combines great stability with precision of adjustment. 



Eight new invar etalons for interferometry have been constructed 

 and adjusted to about one-tenth of a wave-length. Five of them, 

 having lengths of 1.0, 2.5, 5.0, 7.5, and 10.0 mm., respectively, are 

 designed for use on the mountain, and the others, of lengths 2.5, 5.0, 

 and 10.0 mm., for the laboratory. In addition, a new frame has been 

 constructed for supporting the etalons intended for laboratory use. 

 All the etalons are interchangeable. Tests show that with no protection 

 from the usual variations in laboratory temperature these etalons suffer 

 no change greater than 1 part in 10,000,000 in a period of about 2 hours. 



An etalon mounting has been adapted for use at the focal plane of 

 the 60-inch telescope and tested for constancy. Control plates taken 

 6 hours apart — between which the temperature varied about 5° C, 

 the driving-clock was wound three times, and the apparatus was re- 

 moved from the telescope and replaced — showed no loss of adjustment 

 and a length variation of less than 2 parts in a million. 



A Leeds and Northrup high-sensitivity galvanometer has been pur- 

 chased. A further acquisition is a Wolff potentiometer with accessories, 

 purchased by Professor H. S. Carhart on a grant from the Carnegie 



