MOUNT WILSON SOLAR OBSERVATORY. 225 



addition contains two dark-rooms and an office, provides a more 

 favorable installation for the interferometer spectrograph and space 

 for a well to be excavated later for a plane-grating spectrograph of 

 longer focus than that now in use. Three large instrument piers in 

 the new portion are fitted with gas, water, and drain, as well as elec- 

 trical connections, A large switchboard is being constructed, by 

 means of which direct and alternating current at a variety of voltages 

 can be supplied to all parts of the laboratory. 



The auxiliary concave-grating spectrograph has been provided with 

 a permanent mounting. Three concrete piers support the instrument 

 and a substantial double-walled wooden housing affords efficient pro- 

 tection to the entire apparatus. After the adjustment of the instru- 

 ment, curves giving the position of the camera and the focal setting 

 for any wave-length were determined experimental^ and found in good 

 agreement with those calculated from the fundamental constants 

 of the apparatus. 



A new etalon embodying improvements indicated by our experience 

 with the earlier types has been constructed, and the tests made have 

 proved it to be highlj^ satisfactory. The frame is entirely of steel, 

 designed to give extreme stiffness and stability, and will accommodate 

 plates of different thickness with separators of fused quartz up to 

 20 mm. in length. Six of these separators have been completed, having 

 lengths of 2.5, 5, 7.5, 10, 15, and 20 mm. respectively. Under actual 

 working conditions the apparatus is remarkably insensitive to thermal 

 and mechanical disturbances — the whole order of interference is con- 

 stant for a period of many weeks and the corrections necessary from 

 time to time to insure parallelism of the plates are exceedingly minute. 



The adjustment of the etalon is now carried out in situ with the aid 

 of the long-focus mirror that, in actual use, projects the rings upon the 

 slit, and after parallelism has been attained it is unnecessary to touch 

 the instrument. Changes in thickness occurring during a day's obser- 

 vations amount, at a maximum, to only one part in a million. 



A valuable addition to the electric equipment has been made by the 

 purchase of a transformer of 10 kw. capacity, capable of potentials up 

 to 120,000 volts. It is provided with a rotating contact device driven 

 by a synchronous motor by which unidirectional discharges may be 

 obtained; these are especially useful for high vacua. 



A new arc-and-spark chamber has been built in the shop, for opera- 

 ting these sources either in vacuo or at pressures up to 5 atmospheres. 



The small motor-generator set formerly used in the laboratory has 

 been modified by exchanging the old generator giving 125 volts for a 

 new one yielding 250 volts with a full-load output of 15 amperes. This 

 serves admirably for operating arcs for wave-length and other pre- 

 cision purposes, and saves the losses formerly entailed by operating two 

 large generators in series. 



