MOUNT WILSON OBSERVATORY. 245 



(5) freedom from close companion lines; and, finally, to other considera- 

 tions, such as their usefulness in forming a framework for a new table 

 of solar wave-lengths. For economy of time it is planned to confine the 

 measurements for the present to this restricted list of lines, leaving a 

 large amount of material available for future study. 



EFFECTIVE WAVE-LENGTH OF SUNLIGHT. 



In astronomical applications of the interferometer, the results of a 

 measurement will always be given in the form a = KxX, where a is 

 an angular distance, K a quantity determined by the constants of the 

 instrument used, x the setting made, and X the mean or effective 

 wave-length of the light of the body studied. K can always be deter- 

 mined with great accuracy; experience up to date indicates that x 

 can be determined to 1 per cent or less. In order to utilize the fuU 

 efficiency of the instrument, we must therefore know X to a few tenths 

 of 1 per cent. 



Given certain data, such as the spectral-energy distribution of the 

 object under investigation, the transmission of the atmosphere for 

 different wave-lengths, the sensitiveness of the eye to different colors, 

 and a number of optical constants of the instrument employed (diffi- 

 cult to determine), the effective wave-length X can be calculated. 

 At present such a calculation is possible only for the sun, and even 

 here an experimental check is highly desirable on account of the 

 number of constants entering into the calculation. 



An experimental determination of the value X for sunlight after 

 reflection from two clean silver surfaces has been made by Mr. Ander- 

 son, both in Pasadena and on Mount Wilson. The observations on 

 Mount Wilson show that the changes in the effective wave-length 

 between zenith distance 0° and 60° are practically negligible, amount- 

 ing to only 0.3 per cent. Under ordinary observing conditions on 

 Mount Wilson, the wave-length may therefore be regarded as a con- 

 stant for a given object, provided its zenith distance does not exceed 

 60°. The accuracy of the experimental determinations of X for the 

 sun appears to be of the order of 0.1 per cent. 



A method of determining the value of X for any star was also de- 

 vised and tested, but was found to have a probable error of roughly 

 2 per cent, due principally to the effects of seeing. It is unnecessary, 

 however, to employ this method, for since we now know the value 

 of X for the sun, the correction for a given star may be computed with 

 sufficient accuracy from its color-index or from its temperature as 

 given by Wilsing. 



STABILITY OF ATMOSPHERIC LINES. 



The observations of P^rot (C. R. 160, 549, 1915) raised the ques- 

 tion of the constancy of the wave-lengths of the atmospheric lines. 

 He deduced from his observations on a line in the B band of oxygen 



