Fbbkuaey 21, 1902.] 



SCIENCE. 



297 



Selecting the position of the XY plane to 

 be that which coincides with the plane of 

 this moment, we have two homogeneous 

 equations of condition, namely, 



dY , dZ 



dZ 



dX 

 dt 



(6) 



it can be shown that these are equivalent to 

 the one equation Z=^0. We derive there- 

 fore the following: Theorem. The condi- 

 tion Z=0, which Hansen imposes upon 

 his ideal coordinates, means that he selects 

 for the XY plane the plane of the moment 

 of the vector of instantaneous rotation with 

 respect to the point of coordinates 



dX dY 

 dt' dt' 



The Computation of Laplace's Coefficients 

 hy Means of Gylden's j-coefficients : 

 Kurt Laves. 



In this paper it is shown, how the quan- 

 tities h '" and c'" can be determined by 

 means of the definite integrals ,|S' tabu- 

 lated by Gylden in his Hiilfstafeln (after 

 multiplication with functions of the argu- 

 ment a). A comparison is made with the 

 table of Runkle, formerly used for this 

 purpose. 



Astronomical Photography ivith the 40-inch 

 Refractor and the Tivo-foot Reflector of 

 the Yerkes Observatory (Illustrated 

 with lantern slides) : G. W. Ritchey. 

 The 40-inch refractor, which was de- 

 signed for visual observations, has been 

 made available for photography through 

 the use of a color screen and isochromatic 

 plates. The greenish-yellow screen, placed 

 in contact v?ith a plate sensitized for light 

 of this color, permits only those rays to 

 pass for which the object glass is corrected. 

 For all but the briefest exposures a double- 

 slide plate-holder is employed. By means 



of two screws, which move the plate and 

 guiding eye-piece, a star just outside the 

 field being photographed is kept at the 

 intersection of two spider lines throughout 

 the exposure. In this way irregularities 

 in the driving of the telescope, changes in 

 refraction, etc., are corrected. The result- 

 ing photographs of the moon, Orion nebula, 

 star clusters, etc., are exceedingly sharp, 

 and are well adapted for measurement on 

 account of their great scale. The two-foot 

 reflector, on account of its short focal 

 length (8 feet) and its freedom from chro- 

 matic aberration and absorption, is adapted 

 for a different class of work, in which it 

 admirably supplements the 40-inch refract- 

 or. All parts of this reflector were con- 

 structed at the Yerkes Observatory. On 

 account of the perfect driving of the clock, 

 permanence of coUimation of the large 

 mirror and freedom from flexure in the 

 mounting, photographs of excellent defini- 

 tion are easily obtained. Among those ex- 

 hibited Avere the Andromeda nebula, the 

 Orion nebula, nebulai in the Pleiades, and 

 the expanding nebula surrounding Nova 

 Persei. 



A Remarkable Disturbance of the Sun's 

 Reversing Layer: Gboege E. Hale. 

 A series of photographs of the solar spec- 

 trum, taken at the Kenwood Observatory 

 in February, 1894, shows that the reversing 

 layer surrounding a sun-spot was the scene 

 of a great disturbance, which lasted only 

 a few minutes. The diameter of the dis- 

 turbed area was not less than one-sixth that 

 of the sun. Over this entire region the 

 dark lines of the solar spectrum were for 

 a short time so changed in appearance as 

 to be wholly unrecognizable. Measure- 

 ments of the photograph show that nearly 

 all of these lines occur in the normal solar 

 spectrum, and that the changed appearance 

 is due to great changes of relative intensity. 

 Thus numerous lines barely visible on Row- 



