Januakt 28, 1910] 



SCIENCE 



155 



examined numerous hypotheses sufficient to ex- 

 plain the term, in order to clear the ground of 

 those which seemed to be of doubtful value and 

 to bring forward those which appeared sufficiently 

 reasonable to merit tests from observations of a 

 different nature. Some account of three of these 

 hypotheses was presented to the meeting. It was 

 stated that a minute libration of the moon would 

 be sufficient, provided it took place in the moon's 

 equator and had the proper period. The supposi- 

 tion of magnetic attraction practically demanded 

 (a) a periodic change in the magnetic movement 

 of the earth or of the moon. If (a) were re- 

 jected, it was necessary to suppose that the mean 

 place of the lunar magnetic axis was near tlie 

 lunar equator and that the oscillations of its 

 position toolc place in the plane of the equator. 

 The observed secular change of the earth's mag- 

 netic axis could not produce the phenomenon with- 

 out demanding a larger motion of the lunar 

 perigee than observation warrants. On the border 

 line between two sets of hypotheses was a curious 

 fact, namely, that if the period of the solar rota- 

 tion coincided very nearly with one of the prin- 

 cipal lunar periods a minute equatorial ellipticity 

 of the sun's mass was sufficient to explain the 

 term. So far as known, these hypotheses do not 

 conflict with any observed phenomena but they 

 cause some theoretical difficulties. 



4. The International Commission on the Teach- 

 ing of Mathematics was suggested some years ago, 

 but the first steps in its organization were not 

 taken until April, 1908. At that time the Fourth 

 International Congress of Mathematicians, then 

 in session in Rome, empowered Professor Klein, 

 of Gottingen, Sir George Greenhill, of London, 

 and Professor Fehr, of Geneva, to appoint such a 

 commission, and to arrange for it to report at the 

 next congress, to be held at Cambridge in 1912. 

 As a result, three commissioners have been se- 

 lected from each of the leading countries and the 

 work has actively begun. It is expected that each 

 of these countries will submit a very full report 

 of the nature of the work in mathematics, from 

 the kindergarten through the college, with some 

 discussion of the range of advanced work in the 

 universities. In the United States the investiga- 

 tion is carried on by means of fifteen committees, 

 each divided into subcommittees. About two hun- 

 dred and seventy-five people are engaged in the 

 work and the subcommittee reports will be sub- 

 mitted during the present winter. The committee 

 reports will be submitted before the summer of 

 1910, and the national report by Easter, 1911. 



5. Since 1902 the staflT of the Smithsonian As- 

 trophysical Observatory has been engaged in 

 bolometrio measurements of solar radiation to 

 determine the " solar constant," and to note pos- 

 sible variations of solar emission. The measure- 

 ments have been conducted at Washington (sea 

 level), at Mt. Wilson (one mile) and at Mt. 

 Whitney (nearly three miles). When corrected 

 for atmospheric losses by employing Bougner's 

 transmission formula, and reduced to mean solar 

 distance, the average results outside the atmos- 

 phere agree within 2 per cent. On good days at 

 Mt. Wilson or Mt. Whitney the results have a 

 probable error of about .5 per cent. By the con- 

 struction and trial of three copies of a standard 

 pyrheliometer of new design, in which the solar 

 heating is continuously removed by water fiowing 

 about the walls of the hollow receiving chamber, 

 and in which the accuracy of the measurements 

 is checked by introducing known amounts of heat 

 electrically in test experiments, the solar constant 

 may now be expressed absolutely in calories per 

 square centimeter per minute. Definitive reduc- 

 tions are not yet quite complete, but the final 

 solar constant value will not differ 2 per cent, 

 from 1.97 calories per square centimeter per 

 minute. Variations of the solar emission of sev- 

 eral per cent, from the mean value appear not to 

 be uncommon, but during the continuance of the 

 Mt. Wilson observations, prolonged periods of 

 differences of 10 per cent, from tlie mean value, 

 such as were suspected in 1903, have not been 

 observed. 



6. The method described by Professor Very con- 

 sists in matching the two halves of a bright line, 

 seen projected upon a uniformly illuminated back- 

 ground. One half of the line (it may be either 

 the upper or the lower half at will) is a bright 

 line or band in a photograpliic negative of a 

 spectrum crossed by dark absorption lines, or in a 

 positive of a bright-line spectrum. The other half 

 of the line may be, if desired, a line in another 

 spectrum, selected for its general similarity; but 

 the best object for comparison is a slit over an 

 illuminated ground-glass screen with means for 

 the following adjustments: (1) The slit can be 

 varied in width by a micrometer-screw. (2) The 

 illumination of the ground glass can be varied by 

 an optical device employing an iris-diaphragm. 

 (3) Tlie half of the field in which the image of 

 the slit lies can be made to duplicate the other 

 half by altering the illumination of the slit-jaws. 



7. Professor Very believes that attempts to de- 

 duce a law of extinction of light in space, based 



