November 19, 1909] 



SCIENCE 



729 



and highest under the same conditions during 

 summer. The low barometer gives exactly op- 

 posite results. AU these phenomena can be ex- 

 plained as the results of radiation and absorption, 

 especially as modified by condensation and vary- 

 ing amounts of water vapor in the atmosphere. 



A Proposed Method of Studying Solar Radiation, 

 at Great Altitudes: \V. J. Humphreys. 

 The fact that the solar spectrum is limited to 

 wave-lengths greater than 2,900 Angstrom units, 

 makes it desirable to determine whether this 

 limitation is due to atmospheric or to solar ab- 

 sorption. High mountain observations have not 

 definitely settled the question and therefore ob- 

 ser\'ations at much greater elevations would be 

 desirable. It is proposed to send small automatic 

 spectrographs to great altitudes with sounding 

 balloons. A suitable spectrograph for this pur- 

 pose was described with a method for securing 

 proper illumination of the slit and exposure at 

 any predetermined altitude. 



Planetary Magnetism of the Su7i: W. J. Husi- 



PHEETS. 



Assuming an ionization and electric separation 

 in the sun's atmosphere sufficient to account for 

 the magnetic condition that Hale has found in 

 the spots, and assuming unit magnetic permeabil- 

 ity, or that which obtains for all known sub- 

 stances at high temperatures, it is easy to com- 

 pute the magnetic field of the rotating sun as a 

 whole. This would be sufficient to produce a 

 magnetic separation, in the case of the more sen- 

 sitive lines of only about one one-thousandth of 



o 

 an Angstrom unit, an amount too small for cer- 

 tain detection. An absence of measurable polar 

 efiects must therefore not be taken to be in con- 

 flict with the cyclonic theory of the origin of 

 magnetism in the spots. 



Neio Plans for Tabulating the Moon's Longitude : 



E. W. Brown. 



These plans having already been put into more 

 or less definite shape, the paper contained an 

 account of those parts of them which presented 

 unusual features. The main difficulty consists in 

 tabulating the very numerous small terms chiefly 

 due to planetary action. The great majority of 

 these can by special devices be put into tables. 

 It is hoped that a machine which has just passed 

 through the experimental stage will enable the 

 computer to obtain the sum of the other small 

 terms with great rapidity for half-daily intervals. 

 An outline was given of the general principles 

 that were used as guides for forming tables, the 



interests of the ephemeris computer being placed 

 before those of the single place computer whenever 

 they were at variance. A detailed account of the 

 methods will be published within a few months. 

 A Proposed Design for an Objective Prism Spec- 

 trograph for the Determination of Radial 

 Velocities: Frank Sciilesinger. 

 It is proposed to employ two photographic 

 doublets of say six inches aperture and of nearly 

 equal focal lengths. Before each is to be placed 

 a prism of the same aperture, the refracting edge 

 of the one being turned toward the north and of 

 the other toward the south. Plate glass is to be 

 employed for the sensitive plates and one of them 

 is to be turned with the glass side toward the 

 objective. One of the objections to the use of an 

 objective prism in quantitative work is the effect 

 of changes of temperature upon the dispersion of 

 the prisms. It is proposed to obviate this diffl- 

 culty by surrounding the entire spectrograph with 

 a temperature case supplied with an automatic 

 temperature control, the light from the stars be- 

 ing admitted to the objectives by means of two 

 sheets of plane-parallel glass. The plates are to 

 be measured by superimposing them and obtain- 

 ing the distance between corresponding lines in 

 the two spectra of each star. These distances will 

 be affected by the radial velocity and will there- 

 fore enable us to compute the latter. After an 

 investigation into the various distortions that the 

 spectrograph would involve, and of the sources of 

 error to which the measures would be liable, it 

 was concluded that such a spectrograph would be 

 able to determine the radial velocities of faint 

 stars with a probable error not exceeding ten 

 kilometers for each pair of plates. 

 Improvements in the Observatory at A}in Arbor: 



W. J. HUSSEY. 



These consist in the overhauling of the director's 

 residence and the installation of instrument shops, 

 a new 37-inch reflector, a single-prism spectro- 

 graph and a seismograph. 



On Differential Flexure in the Single-prism Spec- 

 trograph : R. H. CcRTiss. 



After calling attention to the serious eflfect that 

 flexure might exercise in this form of spectro- 

 graph. Dr. Curtiss described the device adopted 

 to eliminate it at the Detroit Observatory. As in 

 the case of the Southern Mills three-prism spectro- 

 graph and the Mellon single-prism spectrograph 

 of the Allegheny Observ'atory, the spectrograph 

 box is supported at two points so placed as to 

 make the flexure a minimum. With the Detroit 

 spectrograph Dr. Curtiss has introduced the fur- 



