162 RECENT PROGRESS IN ASTRONOMICAL RESEARCH. 



tion as to the source of the heat of the sun — all these and doubtless 

 other questions of equal interest are still in dispute. 



The Union seeks to foster a spirit of interest in solar research; to 

 suggest needful investigations; to avoid unnecessary duplication of 

 work; to promote, where desirable, uniformity of methods of reduc- 

 tion of observations; and especially to encourage solar observations 

 in regions of the earth where they have hitherto been neglected, so 

 as to tend to keep the sun always under observation, that no unusual 

 and interesting phenomena may escape being recorded. Besides 

 these general activities the following special subjects w^ere discussed 

 and are receiving the attention of expert subcommittees appointed at 

 the Oxford meeting. 



1. Standards of loave length. — The first absolute measures of the 

 wave length of light were made by Sir Isaac Newton about the year 

 1704, although he did not recognize light to be propagated by waves 

 at all, and interpreted his results in accordance with the corpuscular 

 theory. The first advance came with the invention of the diffraction 

 grating by Fraunhofer in 1815, and his measurements of the wave 

 lengths of the principal lines of the solar spectrum were the standards 

 till 1868. In that year Angstrom published the first map of the solar 

 spectrum upon the normal or wave-length scale, basing the wave 

 lengths upon his own determinations. In recognition of his great 

 work, wave lengths have generally been stated since his time in the 

 so-called "Angstrom units," or ten millionths of a millimeter. Thus 

 the wave lengths of the yellow lines of sodium are near 5,890 

 Angstrom units. 



Twenty years later a new era of spectroscopy was made possible 

 by the work of Rowland. After inventing devices for correcting the 

 errors in the cutting of screws, and other devices for correcting the 

 errors which would be introduced in actual ruling by the errors left 

 outstanding in the most perfectly corrected screw, he ruled diffraction 

 gratings of an order of excellence and size till then unapproached 

 and only very recently excelled. He invented also the concave grat- 

 ing, and showed how it might be used in the spectroscope without 

 additional aid of lenses or mirrors to focus the rays. 



There is a characteristic of the grating spectrum which was used 

 by Rowland, Langley, and many others for connecting (as was sup- 

 posed, rigidly) the wave lengths of different sj^ectral rays. This 

 consists in the fact that a large number of spectra are thrown by the 

 grating in the same direction, so that if light of wave length 7,000 

 Angstrom units was found in the spectrum of the first order at a cer- 

 tain point, there would be at the same point light of ^•-°/-», ''-»/-% 3^V-% 

 * * * Angstrom units in the higher orders of spectra. 



Aided by this relation and employing the new and unexcelled con- 

 cave gratings, together with photographic plates of the finest grain 



