1'2 



SEE— DETERMINATION OF THE 



[January 5, 



by Struve to be defined by the equation 



(0.990651 )--S 



(I) 



where x is the distance of the star, in units of A= \/(2.5i2)" and n 

 is the diflference in magnitude. At very great distances nearly all 

 the light is cut off, and it therefore becomes a question of high 

 importance to determine as accurately as possible the proper value 

 for the coefficient of extinction. 



Struve's value, used in the above formula, seems to be too large, 

 and I have therefore calculated a new table, to show the effect of 

 decreasing the coefficient. In justification of this course it should 

 be recalled that Sir William Herschel ignored extinction entirely; 

 but while this procedure obviously is defective, it is pretty clear, 

 from the aspects of the Alilky Way as now made known by modern 

 research, that Struve's coefficient is decidedly too large. The follow- 

 ing table shows the effects of varying the coefficient, upon stars 17 

 magnitudes fainter, corresponding to a distance 2,512 times larger, 

 where x — 1=2,511. 



Table for Varying Coefficient of Extinction. 



From the study of this table, we perceive that at the distance 



