April 19, 1907] 



SCIENCE 



613 



A+aa!+sin tf • 2/+C0S $■ s!+sin 2S. m+cos 2S. «=)i. 



A is a constant correction to the latitude 

 assumed. 



X a uniformly progressive change. 



The period of is six years. 



The solution results in the following ex- 

 pression for A<^ in units of the second deci- 

 mal place: 

 — 1 .96 sin » -I- 0.35 cos e -I- 1 .25 sin 2(9 + 0.94 cos 20, 



X is quite inappreciable. 



The maximum and minimum values of 

 this expression are as follows: 



Maximum 1898, Februaiy 1, + 0".024 



Minimum 1902, April 9, — 0".035 



The range, 0".059, seems too great to be 

 altogether fictitious when the amount and 

 character of the data employed are con- 

 sidered. 

 Period of the Solar Rotation: Philip Fox. 



An investigation of the solar-rotation 

 period based upon measurements of posi- 

 tions of 1,600 calcium floceuli on one hun- 

 dred of the Rumford spectroheliograms 

 taken at the Terkes Observatory in the 

 year 1904, gives the following results: 



The investigation will be continued, 

 using the plates for the years 1905 and 

 1906. 



Opportunities for Solar Research: George 



E. Hale. 



It is safe to say that every astronomer 

 would prize an opportunity to observe any 

 of the fixed stars from a position where its 

 disk would appear as large as the sun. It 

 does not seem probable, however, that such 

 observations of stellar phenomena can ever 



be made, except in the case of the sun itself. 

 For it should ever be borne in mind, when 

 considering the importance of solar re- 

 search, that our most intimate knowledge 

 of steUar phenomena must be derived from 

 solar observations. In the case of the other 

 stars, we may determine their positions, 

 measure their radial velocities, observe 

 their brightness and analyze their light, 

 but we have no means of studying the 

 details of their structure, which must be 

 understood before we can advance far in 

 the solution of the great problem of stellar 

 development. Thus we are driven back to 

 the sun and forced to the conclusion that 

 this typical star well deserves our most 

 serious attention, and the application of 

 every available means of research. 



One can not but be impressed, when con- 

 sidering the sun from this standpoint, with 

 the comparative neglect of the numerous 

 opportunities awaiting the student of solar 

 physics. It is possible, by the application 

 of easily available instruments, for any 

 careful student, wherever situated, to solve 

 solar problems of great importance. If 

 time permitted, it could be shown that al- 

 most all the apparatus required in such 

 work can be constructed at very small ex- 

 pense. For our present purpose, however, 

 let us assume that the observer has at his 

 disposal one of the coelostats so conunonly 

 employed in eclipse work. If this ccelostat 

 has a rather thick mirror, which is fre- 

 quently resilvered, it may be depended 

 upon to serve well for solar work, provided 

 that the mirror is shielded from sunlight 

 during the intervals between the exposures 

 of photographs, and that these exposures 

 are made as short as possible. We may 

 assume that the sunlight is reflected from 

 the ccelostat mirror to a second plane mir- 

 ror (which should also be as thick as pos- 

 sible) and from this mirror to an objective, 

 which should have an aperture of at least 



