IT - -' Scientific Intelligence. 



small book by the Chief Science Master of St. Paul's School, 

 England, describes a few elementary chemical demonstrations, 

 gives simple laboratory experiments and questions connected 

 with them, and gives a few notes describing chemical substances 

 and processes. Tbe course laid out is exceedingly elementary, 

 and no chemical symbols or equations are employed. It deals 

 with facts and omits theories. As far as it goes, however, the 

 book is clearly written and presents interesting experiments. 



H. L. W. 



5. Publications of the Yerkes Observatory, Vol. HI, Parts I 

 and II. Chicago, 1903 and 1907 (University of Chicago 

 Press). — Part I is entitled "The Rumford Spectroheliograph of 

 the Yerkes Observatory, " by George E. Hale and Ferdinand 

 Ellerman. The spectroheliograph was invented in 1889 and 

 it was designed for the study of the chromosphere and promi- 

 nences. Since instruments of this kind do not form a part of 

 the regular equipment of physical laboratories and as the Rum- 

 ford spectroheliograph is a specially fine example of this type 

 of apparatus, it may not be superfluous to give a brief account 

 of the salient features and chief dimensions of this accessory of 

 the 40 in. Yerkes refractor. 



A spectroheliograph is essentially a monochromatic illumi- 

 nator so arranged as to photograph the sun's image by continu- 

 ous successive exposures taken after the fashion of a very 

 narrow focal-plane shutter. With large instruments it is not 

 feasible to follow the ideal plan of translating the spectrohelio- 

 graph bodily across the solar image, hence the alternative 

 equivalent method of simultaneously moving this image across 

 the collimator slit and the photographic plate across the camera 

 slit has to be followed. The diameter of the solar image formed 

 by the 40 in. refractor is 7 in. so that the curved slits of the 

 spectroheliograph have the uncommonly great length of 8 in. 

 For economic reasons the collimator and camera objectives of the 

 Rumford instrument are Voigtlander portrait lenses of linear 

 aperture 6-25 in. and equal focal lengths. The favorable loca- 

 tion of the declination motor on the great telescope caused it to 

 be used, instead of the right ascension motor, to effect the simul- 

 taneous translation of the solar image and the photographic 

 plate. Since the curve connecting the focal length of the 

 refractor with the color is very steep in the violet and ultra- 

 violet, the spectroheliograph is so mounted that it can be racked 

 as a whole along the optic axis of the telescope, thus enabling 

 sharp images of a chosen spectral line to be thrown on the colli- 

 mator slit. This unilateral slit can be rotated around an axis 

 perpendicular to the plane of its jaws until the tangent line 

 at the center of its length becomes parallel to the refracting 

 edges of the prism train or to the rulings of a plane grating. 

 Since the focused heat from the sun would cause the jaws of this 

 slit to expand and close at the center it was necessary to fix 



