j --581 Star-gauging 333 



clustering of the stars in space, Herschel chose the former 

 of these two hypotheses; that is, he treated the apparent 

 density of the stars on any particular part of the sky as 

 a measure of the depth to which the sidereal systems 

 extended in that direction, and interpreted from this point 

 of view the results of a vast series of observations. He 

 used a 2o-foot telescope so arranged that he could see 

 with it a circular portion of the sky 15' in diameter (one- 

 quarter the area of the sun or full moon), turned the telescope 

 to different parts of the sky, and counted the stars visible 

 in each case. To avoid accidental irregularities he usually 

 took the average of several neighbouring fields, and published 

 in 1785 the results of gauges thus made in 683* regions, 



FIG. 83. Section of the sidtreal system. From Herschel's paper in 

 the Philosophical Transactions. 



while he subsequently added 400 others which .he did not 

 think it necessary to publish. Whereas in some parts of- 

 the sky he could see on an average only one star at a time, 

 in others nearly 600 were visible, and he estimated that 

 on one occasion about 116,000 stars passed through the 

 field of view of his telescope in a quarter of an hour. 

 The general result was, as rough naked-eye observation 

 suggests, that stars are most plentiful in and near the 

 Milky Way and least so in the parts of the sky most remote 

 from it. Now the Milky Way forms on the sky an ill- 

 defined band never deviating much from a great circle 

 (sometimes called the galactic circle) ; so that on Herschel's 

 hypothesis the space occupied by the stars is shaped 

 roughly like a disc or grindstone, of which according to 



* In his paper of 1817 Herschel gives the number as 863, but a 

 reference to the original paper of 1785 shews that this must be a 

 printer's error. 



