206 H. Crew — Period of Rotation of the Sun. 



(d) The observation of faculse, giving the period for a layer 

 probablv a few hundred miles higher than the photosphere. 

 Present data extend from 24° S. to 33° K 



Each of these methods gives the synodic period. 

 The object of the work communicated in this paper is to 

 measure the rotation for a zone some 60° wider than any 

 hitherto observed, and to verify or disprove the conclusion 

 which appeared to follow from some observations made a year 

 ago.* The measurements then at hand gave for the daily angu- 

 lar motion, 6, of any point on the sun's absorbing layer, as a 

 function of the heliocentric latitude, £, 



6 — 838' (1 + 0-00335 X), 



which makes the velocity a minimum at the equator, the place 

 at which, for sun spots, it is a maximum. 



There was also, apparently, a systematic errorf in the first 

 series, depending either upon the heating of the spectroscope 

 or upon some cause which varied with the date of observation. 

 This, I hope, has been eliminated in the second set of measure- 

 ments, which are here given. 



They were made with the same instrument, viz : the large 

 spectrometer of the Johns Hopkins University, but with the 

 three modifications which follow. 



The method of shifting the sun's image across the slit was 

 so changed that almost any solar latitude could be observed at 

 any time. 



This was accomplished by a device suggested by Professor 

 Rowland. The brass ring which held the condensing lens 

 was furnished with a metal arm, not unlike the handle of a 

 palm-leaf fan. This arm was pinned to a brass collar, larger 

 than the lens and surrounding it. This pin was the axis, 

 parallel to the optical axis of the lens, about which the lens 

 rotated through a small arc, throwing the image of the sun, 

 now with its eastern, now with its western, limb on the slit. 

 But this collar, besides having adjustable stops to limit the 

 motion of the lens, could also be rotated in its own plane, and 

 clamped in any azimuth, thus changing the direction of the 

 motion of the image across the slit. To make this latter 

 change was the only purpose of this part of the apparatus. 

 For a reflecting prism was inserted between the condensing 

 lens and the collimator, and by rotating this, any desired 

 portion of the sun's limb could be made tangent to the slit. 

 This having been done, the image would, in general, no longer 

 move at right angles to the slit ; so the device above described 

 was used to counterbalance the effect of the prism on the 



*Crew, this Journal, Feb.. 1888. 



f Discussion of this in Observatory, April, 1888. 



