1872.) Notices of Books. 383 
at every point of the surface. The results obtained are de- 
finite, and we commend them to the notice of our mathematical 
readers. 
Index of Spectra. By Wititam Marsnatt Watts, D.Sc., 
Senior Physical Science Master in the Manchester Grammar 
School. With a Preface by H. E. Roscoz, B.A., Ph.D., 
F.R.S., Professor of Chemistry in Owen’s College, Man- 
chester. London: Henry Gillman. 1872. 
Dr. Roscog, in the preface, alludes to the inconvenience arising 
from the employment of different scales in mapping spectra. 
Spectroscopists generally will agree with him. Nothing is more 
annoying than to find, after collecting the measurements scat- 
tered through many volumes, that before the measures are com- 
parable they must be reduced to a common standard. This 
difficulty will no longer exist. Dr. Watts has much facilitated 
spectroscopic research, by collecting all existing measurements 
of the spectra of the elements, and presenting them on a uniform 
scale of wave-lengths, upon the basis of Angstrém’s measure- 
ments of the wave-lengths of the principal Fraiinhofer lines. 
This basis is without doubt the most accurate, for Angstrém’s 
measurements are so very exact that it is unlikely that any cor- 
rections which may be rendered necessary by new and more 
exact measurements will affect them, except in the decimal 
place. Besides Angstrém’s numbers, however, Dr. Watts gives 
those obtained by Fraiinhofer, Ditscheiner, Bernard, Van de 
Wiliigen, Mascart, and Esselbach. Of course we have the 
mapping of the spectra of the elements by Huggins, Thalén, 
and Kirchhoff; and where these three savants have not endorsed 
the examination of the spectrum, in each case reference is made 
to the original memoir. Mascart, Ketteler, and Miller’s num- 
bers were obtained by direct observation of the diffraction 
spectra. The measurements for chlorine, bromine, iodine, phos- 
phorus, sulphur, selenium, nitrogen, and oxygen, are those by 
Plicker, and are reduced to wave- ‘lengths by means of an inter- 
polation curve drawn from the lines of oxygen and nitrogen. 
The measurements and calculations have been verified, as eS as 
possible, by Dr. Watts, upon his own spectroscope, while 
twenty-eight of the results are checked by comparison with those 
of Dr. Gibbs. The numerical results are well assisted by the 
graphic illustrations, in which, on Bunsen’s plan, the intensity 
of the line is indicated by the height. A complete set of chromo- 
lithographs, of the double spectra of nitrogen, sulphur, and 
carbon, is appended. Besides the method of graphic interpola- 
tion, Dr. Gibbs’s interpolation and extrapolation formule have 
been given, with simple and clearly-explained examples. We 
are sure that every spectroscopist will agree with us when we 
say, sincerely is Dr. Watts to be thanked for the laborious task 
