MOUNT WILSON SOLAR OBSERVATORY. 271 



to be attributed to the non-maxwellian distribution of the velocities, 

 the large velocities being in excess. Whether, as a second cause, we 

 shall have to admit that the velocities of the more luminous stars are 

 less than those of the fainter stars, remains yet to be seen. Only the 

 K stars have as yet been investigated in regard to this point. The 

 result is, however, not decisive. 



PHYSICAL LABORATORY. 

 INSTRUMENTS. 



A concave-grating spectrograph of 15-foot radius, mounted vertically 

 in the laboratory well and combining the useful features of the Rowland 

 mounting with close temperature control, was completed in October 

 1913. With it the spectrum may be photographed from X 2000 in the 

 first order (scale 3.7 1 per mm.), to X 7000 in the second order. The 

 5-inch grating, ruled by J. A. Anderson, is of high quality. For the 

 study of the ultra-violet and the photography of extended regions of 

 wave-length with moderate dispersion, the instrument supplements the 

 facilities offered by the plane-grating spectrograph. 



The fittings of the electric furnace have been improved so that the 

 temperature may be held nearly constant during long exposures. Pro- 

 vision has been made for the operation of an arc within the furnace 

 chamber, which permits a study of the arc in vacuo and a comparison 

 with the effects given by the vacuum furnace. Various atmospheres 

 about the arc and high pressures may also be employed. 



The 50-k.w. transformer used for the furnace and ''tube-arc" is 

 being replaced by one of 100 k.w. giving voltages in 5-volt steps from 

 5 to 50 volts. This will permit the use of larger furnace tubes and 

 give greater flexibility. 



The larger articles of equipment purchased are a Gaede oil-pump for 

 moderate vacua, a Leeds and Northrup testing-set and moving-coil 

 galvanometer, a Wolff decade resistance, ratio resistance, standard 

 resistance of 100 and 1,000 ohms, and a 4-inch plane grating ruled by 

 Anderson, giving high intensitj^ in the first-order spectrum. 



Mr. St. John and Mr. Babcock have devised apparatus for simul- 

 taneously photographing spectra of the same source, or different parts 

 of the same source, which has proved of great value in eliminating small 

 instrumental shifts. By means of prisms the exposures are made 

 rigorously simultaneous, differences in intensity being compensated by 

 a rotating sector of variable opening. 



For use in the measurement of strong magnetic fields Mr. Babcock 

 has designed and constructed 8 test coils and 2 mutual inductances. 

 The magnetic areas of the coils have all been calculated. He has also 

 constructed a nickel resistance thermometer and adapted the Callendar 

 recorder to show with it very small temperature variations in the ruling- 



