152 JS. Crew — Rotation of the Sun. 



the sun, when examining an equatorial diameter, and the 

 absence of any displacement when comparing the extremities 

 of the polar diameter, and has also commented on this as a 

 means of separating the solar and telluric lines. 



In 1876, Prof. Young* published the first accurate measure- 

 ments of the linear velocity of a point on the solar surface. 

 These were made with a spectroscope attached to a 9£ inch 

 equatorial, and fitted with a Rutherfurd grating, ruled with 

 8640 lines to the inch. His result is for a mean latitude of 7° 

 and is reduced to the equator by Faye's formula for the pioper 

 motion of sun spots. The velocity thus obtained, 1'42 

 miles per second, is so large as to suggest a physical 

 significance, viz : that the reversing layer, whatever and 

 wherever it may be, has a greater angular velocity than that 

 layer of the solar surface in which the sun spots lie, a conclu- 

 sion very interesting considering that the observations of De la 

 Rue,f Stewart, and Loewy at Kew appeared to show a lagging 

 behind of that layer of the solar atmosphere in which the 

 faculse occur, a layer probably at no inconsiderable distance 

 above the altitude of sun spots. 



Carrington,^: by a magnificent series of laborious observa- 

 tions, extending over seven years, has determined what maybe 

 called the proper motion of the photosphere, or more strictly, 

 the law according to which sun spots move relatively to one 

 another in latitude and longitude. 



Have the different parts of the reversing layer a relative 

 motion, and if so, what is the law which governs it % 



It was in the hope of obtaining at least a partial answer to 

 this question that the following observations were made. 



Apparatus. — The light was furnished by an equatorial 

 heliostat with an auxiliary plane glass mirror. Leaving the 

 heliostat, the ray next fell upon a condensing lens of 8 cm 

 diameter and 135 cm focal length, thus giving upon the slit of 

 the collimator, an image 12 - 5 mm in diameter. For the follow- 

 ing apparatus to move the image across the slit, I have to thank 

 the kindness and skill of Prof. Rowland. The condensing lens 

 was screwed into a rectangular wooden frame which was 

 fastened at the bottom by a single bolt to a larger iron frame. 

 This bolt was parallel to the optical axis of the lens, and the 

 lens could be made to rotate about it through an arc of 5 or 10 

 degrees, the*arc being limited by adjustable stoj)s. The iron 

 frame was, in addition, capable of lateral motion, while the lens 

 in the wooden frame could also be adjusted vertically. These 

 adjustments enabled the observer to bring either limb of the 



* Young: this Journal, xii, 321, 1876. f Proc Hoy. Soc, xiv, 37. 



X Carrington: Observations on solar spots, London, 1863. 



