MOUNT WILSON OBSERVATORY. 217 



separations of about 20 feet and 12 feet, respectively, corresponding 

 to angular diameters of 0!022 and 0''040, and linear diameters of about 

 21,000,000 and 400,000,000 miles. In the case of several other stars, 

 evea when the angular diameter is much smaller, distinct decrease in 

 visibility has been observed when the mirrors were separated. In fact. 

 Professor Michelson has made very precise estimates of decreasing 

 visibility for white stars, and has devised a simple auxiliary appa- 

 ratus for producing comparison fringes of knowai visibility. Thus, 

 if uncertainties due to variations in seeing can be allowed for, which 

 is still uncertain, it may become possible with the 20-foot inter- 

 ferometer to obtain approximate measures of stars of very small 

 angular diameter. 



The importance of this investigation is obvious, as it has already 

 given beautiful confirmation of Russell's theory, proving the existence 

 of giant stars of enormous size and of mean density about one- 

 thousandth that of atmospheric air. The new method will be developed 

 to the fullest possible extent and applied to all the stars that come 

 within its range. 



The third very exceptional event in the progress of the year dates, 

 in its inception, from the origin of the Observatory. At Kenwood on a 

 small scale and at the Yerkes Observatory on a larger one, the writer 

 had utilized laboratory experiments as the necessary means of inter- 

 preting astrophysical phenomena. In this respect he simply had 

 followed the example of Huggins, Lockyer, and others, gradually in- 

 creasing the scale of the instrumental equipment as need and experience 

 developed. A physical laboratory naturally formed part of the original 

 Mount Wilson equipment, and this has proved so necessary to the inter- 

 pretation of solar and stellar phenomena that it is now being advanced 

 from a secondary to a primary place in the scheme of the Observatory. 

 The importance of taking this step was mentioned in the last two an- 

 nual reports, and progress has already been made, as the present 

 report indicates, in building up the equipment on the necessary scale. 



From the outset this equipment has been planned with special 

 reference to the practical problem of interpreting astronomical phe- 

 nomena. While it has not been thought wise to limit our physical in- 

 vestigations to such needs, it has seemed equally inadvisable to expand 

 very far into the domain of the investigator who deals primarily with 

 the fundamental laws of physics and chemistry. Thus, in our work on 

 the Zeeman effect, begim for the express purpose of interpreting sun- 

 spot phenomena, it has seemed best not to limit our laboratory studies 

 simply to the lines affected in spots, but also to include other lines and 

 to extend the range of wave-length well into the ultra-violet, beyond 

 the termination of the characteristic spot spectrum. Similarly, in the 

 Stark effect and now in the combined effect of electric and magnetic 

 fields on radiation, we have not adhered narrowly to the viewpoint of 



