138 • ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



Stebbins uses for this purpose the variable resistance which selenium 

 offers to the passage of the electrical current when the selenium is 

 more or less exposed to light. But the results do not appear to be 

 regular except when the selenium is kept at low temperatures. 

 Messrs. Elster, Geitel, and Guthnick have utilized the property which 

 certain alkaline metals, such as sodium and calcium, offer of emitting, 

 under the influence of light, corpuscles capable of acting upon an 

 electrometer. They have thus obtained a sensitiveness of one part in 

 a thousand in estimating the brightness of faint stars. 



The photographic study by Prof. Bailey of the cluster Messier 3 

 has shown the existence in this single group of 137 variable stars, all 

 .of the same type and having periods of about a half a day. Stars 

 showing such rapid changes are rarely found outside of clusters. 

 There has, however, meanwhile been found a new example in the star 

 ER Lyrse, investigated by Kiess. 



In order to establish a homogeneous system of magnitudes in a 

 photographic catalogue there has been used with success at the 

 Greenwich Observatory a diffraction grating formed of metallic wires 

 stretched across the front of the objective of the telescope. Each 

 star then furnishes a central image with a series of secondary images 

 on either side. The ratio of the brightness of the successive members 

 of each series can be calculated with precision if a micrometrical 

 study is made of the grating and the widths of the wires and spaces 

 are made very uniform. Each bright star thus will give in the field 

 of the telescope a scale of magnitudes to which the fainter stars may 

 be referred. Messrs. Chapman and Melotte have thus been able to 

 make a complete catalogue of the stars down to the fifteenth magni- 

 tude and within a radius of 25' of the pole. 



The examination made by Reynolds of the distribution of bright- 

 ness in the great Andromeda nebula makes it seem as though a great 

 part of that nebula's brightness were due to a central star too envel- 

 oped and obscured in the diffused matter for us to see it. In the 

 spectrum of this same nebula, generally held to be continuous with a 

 few absorption lines, Messrs. Fox and Max Wolf have found bright 

 lines, and in the spectrum of the Wolf-Rayet stars, characterized by 

 bright lines, Max Wolf finds also the lines of gaseous nebulae. We 

 are thus forced to believe that bright lines are a general characteris- 

 tic of true nebulae, which do not shine by reflected stellar light, and 

 that the Wolf-Rayet stars show a transition type between such nebulae 

 and ordinary stars. 



How is the evolution from one to the other effected? Nicholson 

 has tried to determine it by a subtle analysis of the numerical values 

 of the wave lengths. The only terrestrial elements laiown with cer- 

 tainty as existing in the nebulae, and accordingly in the Wolf-Rayet 

 stars, are hydrogen and helium. The other lines which are in the 



