September 13, 1901.] 



SCIENCE. 



401 



The width of the shadow-path is about 59. S"" 

 and the velocity of the shadow 2900" an 

 hour. The eclipse will be visible from 

 Chamberlin Observatory, Denver, and Mt. 

 Arapahoe, Grand Co. 



Sept. 10, 1923. An eclipse of duration 

 3" 24' is total from San Diego, Cal., east- 

 ward along a line near the United States 

 and Mexican boundary. Width of shadow- 

 path 102 miles. 



Jan. 24, 1925. An eclipse visible (as 

 total) from northern Michigan to New 

 Haven, Ct., reaching the latter point at 9" 

 5"" A.M., and lasting 2° 8\ 



Two maps of the Denver eclipse were 

 shown. 



' Bibliography of Quaternions and Allied 

 Systems of Mathematics ' : Professor Alex- 

 ander Macearlane, Lehigh University. 



The association for the promotion of 

 quaternions and allied mathematics has 

 in preparation a bibliography of all the 

 literature of the subject. The field em- 

 braces all that has been written on what is 

 called geometric algebra, or space analy- 

 sis, and its three main subdivisions are 

 quaternions, Ausdehnungslehre, and geo- 

 metric algebra previous to Hamilton and 

 Grassmann. The extent of this literature is 

 not so small as is commonly supposed. As 

 regards the first branch, the papers of 

 Hamilton himself are numerous, and he 

 has been followed by about one hundred 

 writers. The writings of Grassmann are 

 also numerous, and he has been followed by 

 about the same number of writers. 



It is desired to make the bibliography as 

 complete as possible, and the writer desires 

 mathematicians to cooperate by sending to 

 him the necessary data about their own 

 writings, or any rare writings in their pos- 

 session. 



* The Bruce Micrometer ' : Professor C. 

 J. Ling, Denver Manual Training High 

 School. 



The filar micrometer attached to the 



telescope at the Chamberlin Observatory at 

 University Park, Colorado, is the gift of the 

 late Miss Caroline "W. Bruce. It is espe- 

 cially adapted to rapid work and work on 

 faint objects. Instead of being screwed to 

 the tail piece of the telescope, it is so at- 

 tached that it slides off, thus leaving the 

 zero of the position circle unchanged. The 

 position circle is furnished with a system 

 of solid stops which enable it to be turned 

 exactly 90° when micrometric measures of 

 both Ja and Ad are being made. These 

 stops can be rapidly thrown back, enabling 

 the circle to be turned any desired angle, 

 and so rapidly thrown exactly to their 

 former position. The circle can also be 

 clamped at any position and adjusted by 

 the use of a tangent screw. 



The eye piece can be moved rapidly in 

 both E. A. and Decl., admitting of the use 

 of exceptionally long and a large number 

 of spider webs. It is possible to observe 

 objects diflering in declination by more 

 than 30', and this with a comparatively 

 small motion of the micrometer. A micro- 

 meter screw of 20 threads to the inch makes 

 rapid work possible. This is provided with 

 three heads which are graduated so as to 

 be read rapidly, two of which may be 

 clamped in. position. 



The field is illuminated by two pairs of 

 two candle power electric lamps whose in- 

 tensities are regulated by a rheostat and 

 mechanical shades, making it possible to 

 rapidly adjust the illumination or to throw 

 it off entirely. The observer can also illu- 

 minate at will either or both of the verniers 

 of the position circle. 



The results obtained from many hun- 

 dreds of observations demonstrate that the 

 micrometer is admirably adapted to com- 

 bine rapidity and accuracy, making it an 

 extremely eflScient instrument, and this is 

 all the more worthy of mention since it is 

 accomplished without undue haste on part 

 of the observer. 



