I912-] DEPTH OF THE MILKY WAY. 15 



before many years elapse. This would be completing on a modern 

 scale the sidereal soundings left somewhat incomplete by the sys- 

 tematic explorations of the Herschels. 



In a private letter, written in response to my recent inquiry re- 

 garding the power of the 6o-inch reflector of the Solar Observatory 

 at Mt. Wilson, Professor W. S. Adams, the Acting Director, informs 

 me that this fine instrument probably will show visually stars as faint 

 as 1 8th magnitude. He points out, however, that the magnitude 

 scale is not well defined for such faint objects, and that very few 

 astronomers have enough experience to fix it at the present time. 



Adams also informs me that from a photograph of the region 

 of the northern celestial pole of four hours' duration. Professor 

 E. C. Pickering has derived a value of 21.0 magnitude for the faint- 

 est stars, by the system of photographic magnitudes in use at the 

 Harvard College Observatory. Obviously there is some uncertainty 

 in this value, but it probably is not extreme. 



In answer to an inquiry as to the possibility of getting still 

 fainter stars by prolonging the exposure, Professor Adams assures 

 me that it can be easily done, the only limit being the brightness of 

 the background of the sky; but that with the clear air of Mt. Wilson 

 this would not be reached till the exposure had extended over many 

 hours. He adds that it takes about three times the exposure to 

 obtain a star one magnitude fainter. From the data here supplied 

 it seems certain that stars as faint as 21.0 magnitude may be photo- 

 graphed at Mt. Wilson, with the 60-inch reflector, and that by pro- 

 longing the exposure several additional hours or through whole 

 nights, stars of 22.0 magnitude probably could be obtained. 



It is therefore well established that stars 17 magnitudes fainter 

 than the 225 helium stars, with average magnitude of 4.14, recently 

 investigated at Lick Observatory, may now be photographed with 

 more than one instrument; and the value of A = 2,5 12 used in our 

 calculations is amply justified. In fact it seems probable that instead 

 of 2,512 as our distance multiplier for stars 17 magnitudes fainter, 

 we might have used the larger value 3,981, corresponding to stars 

 18 magnitudes fainter than our 225 helium stars with average magni- 



