MOUNT WILSON SOLAR OBSERVATORY. 1 71 



of the stars known to belong to the Orion type. This fact in itself seems to 

 furnish sufficient reason for the inclusion of all of the Orion stars not under 

 observation elsewhere, for which we have accurate determinations of the 

 proper motion, in the radial velocity program of the 6o-inch reflector. The 

 values of the radial velocities will be used not only in the determination of 

 the distances of the stars, but also to complete the data necessary for an 

 accurate computation of the elements of the star systems. The faintest of 

 these stars will be observed with the low-dispersion spectrograph ; but higher 

 dispersion can probably be used to advantage on the great majority, and for 

 this purpose the large three-prism spectrograph will be employed. 



PHYSICAL LABORATORY. 



The work in the physical laboratory in Pasadena has been chiefly a con- 

 tinuation of the investigations in progress last year, the main lines of work 

 being the study of the effect of a magnetic field upon the light producing 

 the spectrum-lines, carried on by Mr. King and Mr. Babcock; electric- 

 furnace investigations by Mr. King, and a study of the influence of pressure 

 upon spark spectra by Mr. Gale. 



The purpose of the magnetic field work has been to make a study of the 

 Zeeman effect for a number of elements whose lines are prominent in sun- 

 spot spectra. These spectra are photographed with high dispersion and a 

 field of 16,000 to 20,000 gausses through the range from A 3700 to A 6700. 

 The aim has been to tabulate the material in as convenient a form as possible 

 for purposes of comparison with sun-spot spectra and for general physical 

 work. The character of the separation produced by the magnetic field has 

 been determined for each line in so far as the strength of field available has 

 permitted. To do this, separate photographs are taken of the Zeeman com- 

 ponents produced by light vibrations respectively perpendicular and parallel 

 to the direction of magnetic force. The measurement of the separation of 

 the components into which a single line is resolved, frequently several in 

 number, has given a large quantity of material for direct comparison with 

 lines showing separations in sun-spot spectra. In this way the character 

 and strength of the magnetic field present in different sun-spots and in dif- 

 ferent regions of the same spot can be studied most effectively. Further- 

 more, the measurement in the laboratory of the separations of lines under a 

 constant field, taken through a wide range of wave-length, shows in how far 

 these separations are multiples of a fundamental interval, and how the aver- 

 age separation changes with wave-length. 



The results of the detailed study of iron and titanium are fully worked 

 out and will be published during the autumn. The investigation of the 

 spectra of chromium, vanadium, and nickel along similar lines is in progress. 



A comparison by Mr. King of the Zeeman separations for the lines of 

 iron, chromium, and titanium, with the displacements of the same lines under 



