4 Notes on Star- Streams. 



present entertained respecting the Yia Lactea. The elder 

 Herschel, whose nobly speculative views of nature were 

 accompanied by practical common sense, and a wonderful 

 power of patient observation, applied to the heavens his now 

 celebrated method of gauging. He assumed as a first principle, 

 to be modified by the results of observation, that there is a 

 tolerable uniformity in the distribution of stars through space. 

 Directing his twenty-feet reflector successively towards different 

 parts of the heavens, he counted the number of stars which 

 were visible at any single view. The field of view of this 

 reflector was 15' in diameter, so that the portion of the sky 

 included in any one view was less than one-fourth of that 

 •covered by the moon. He found the number of stars visible 

 in different parts of the heavens, in a field of view of this size, 

 •to be very variable. Sometimes there were but two or three 

 stars in the field; indeed, on one occasion he counted only 

 "three stars in four fields. In other parts of the heavens the 

 whole field was crowded with stars. In the richer parts of the 

 galaxy as many as 400 or 500 stars would be visible at once, 

 -and on one occasion he saw as many as 588. He calculated 

 that in one quarter of an hour 116,000 stars traversed the field 

 of his telescope, when the richest part of the galaxy was under 

 ■observation. Now, on the assumption above-named, the 

 number of stars visible when the telescope was pointed in any 

 given direction was a criterion of the depth of the bed of stars 

 in that direction. Thus, by combining a large number of 

 observations, a conception — rough, indeed, but instructive — 

 might be formed of the figure of that stratum of stars within 

 which our sun is situated. 



One section of the galactic nebula, as determined from 

 Herschers observations, is given in Figure 3. The projections 

 extending to the left correspond to those portions of the 



Fig. 3. 



particular great circle considered, which cross the double part 

 of the Milky Way ; the opposite projection represents the 

 portion crossed by the single stream; while the comparative 

 flattening of the central part indicates the gradual diminution 

 of star-density in directions removed from the galactic zone. 

 It is, of course, to be understood that Herschcl was far from 



