SCIENCE 



NEW YORK, OCTOBER 16, 1891. 



CELESTIAL PHYSICS.' 



I DO not purpose to attempt a survey of the progress of 

 spectroscopic astronomy from its birth at Heidelberg in 1859, 

 but to point out what we do know at present, as distin- 

 guished from what we do not know, of a few only of its more 

 important problems, giving a prominent place, in accordance 

 with the traditions of this chair, to the work of the last year 

 or two. 



In the spectroscope itself advances have been made by 

 Lord Rayleigh by his discussion of the theory of the instru- 

 ment, and by Professor Rowland in the construction of con- 

 cave gratings. 



Lord Rayleigh has shown that there is not the necessary 

 connection, sometimes supposed, between dispersion and re- 

 solving power, as, besides the prism or grating, other details 

 of construction and of adjustment of a spectroscope must be 

 taken into account. 



The resolving power of the prismatic spectroscope is pro- 

 portional to the length of path in the dispsrsive medium. 

 For the lieavy flint glass used in Lord Rayleigh's ex- 

 periments, the thickness necessary to resolve the sodium 

 lines came out 1.02 centimetres. If this be taken as a unit, 

 the resolving power of a prism of similar glass will be in the 

 neighborhood of the sodium lines equal to the number of 

 centimetres of its thickness. In other parts of the spectrum 

 the resolving power will vary inversely as the third power 

 of the wave-length, so that it will be eight times as great in 

 the violet as in the red. The resolving power of a spectro- 

 scope is therefore proportional to the total thickness of the 

 dispersive material in use, irrespective of the number, the 

 angles, or the setting of the separate prisms into which, for 

 the sake of convenience, it may be distributed. 



The resolving power of a grating depends upon the total 

 number of lines on its surface and the order of spectrum in 

 use, about 1,000 lines being necessary to resolve the sodium 

 lines in the first spectrum. 



As it is often of importance in the record of observations 

 ■ to state the efficiency of the spectroscope with which they 

 were made, Professor Schuster has proposed the use of a unit 

 of purity as well as of resolving power, for the full resolv- 

 ing power of a spectroscope is realized in practice only when 

 a sufficiently narrow slit is used. The unit of purity also 

 is to stand for the separation of two lines differing by one- 

 thousandth of their own wave-length, about the separation 

 of the sodium pair at D. 



A further limitation may come in from the physiological 

 fact that, as Lord Rayleigh has pointed out, the eye, when 

 its full aperture is used, is not a perfect instrument. If we 

 wish to realize the full resolving power of a spectroscope, 

 therefore, the emergent beam must not be larger than about 

 one-third the opening of the pupil. 



Up to the present time the standard of reference for nearly 

 all spectroscopic work continues to be Angstrom's map of 



' iDau^ral address at the meeting of the British AssociatioQ for the Ad- 

 vancement of Science, at Cardiff, August, 1891, by William Haggins, president 

 of the association (Nature, Aug. 20). 



the solar spectrum, and his scale based'upon his original de- 

 terminations of absolute wave-length. It is well known, as 

 was pointed out by Thalun in his work on the spectrum of 

 iron, in 1884, that Angstrom's figures are slightly too small, 

 in consequence of an error existing in a standard metre used 

 by him. The corrections for this have been introduced into 

 the tables of the wave-lengths of terrestrial spectra collected 

 and revised by a committee of this association from 1885 to 

 1887. Last year the committee added a table of corrections 

 to Rowland's scale. 



The inconvenience caused by a change of standard scale 

 is, for a time at least, considerable; but there is little doubt 

 that in the near future Rowland's photographic map of the 

 solar spectrum, and his scale based on the determinations of 

 absolute wave length by Pierce and Bell, or the Potsdam 

 scale based on original determinations by Miillerand Kempf, 

 which differs very slightly from it, will come to be exclu- 

 sively adopted. 



The great accuracy of Rowland's photographic map is due 

 chiefly to the introduction by him of concave gratings, and 

 of a method for their use by which the problem of the deter- 

 mination of relative wavelengths is simplified to measures 

 of coincidences of the lines in different spectra by a mi- 

 crometer. 



The concave grating and its peculiar mounting, in which 

 no lenses or telescope are needed, and in which all the spec- 

 tra are in focus together, formed a new departure of great 

 importance in the measurement of spectral lines. The valu- 

 able method of photographic sensitizers for different parts of 

 the spectrum has enabled Professor Rowland to include in 

 his map the whole visible solar spectrum, as well as the 

 ultra-violet portion as far as it can get through our atmos- 

 phere. Some recent photographs of the solar spectrum, 

 which include A, by Mr. George Higgs, are of great techni- 

 cal beauty. 



During the past year the results of three independent re- 

 searches have appeared, in which the special object of the 

 observers has been to distinguish the lines which are due to 

 our atmosphere from those which are truly solar — the maps 

 of M. Thollon, which, owing to his lamented death just be- 

 foi-e their final completion, have assumed the character of a 

 memorial of him; maps by Dr. Becker; and sets of photo- 

 graphs of a high and a low sun by Mr. McClean. 



At the meeting of this association in Bath, M. Janssen 

 gave an account of his own researches on the terrestrial lines 

 of the solar spectrum which owe their origin to the oxygen 

 of our atmosphere. He discovered the remarkable fact that, 

 while one class of bands varies as the density of the gas, 

 other diffuse bands vary as the square of the density. These 

 observations are in accordance with the work of Egoroff and 

 of Olszewski, and of Liveing and Dewar on condensed oxy- 

 gen. In some recent experiments Olszewski, with a layer of 

 liquid oxygen thirty millimetres thick, saw, as well as four 

 other bands, the band coincident with Fraunhofer's A, a re- 

 markable instance of the persistence of absorption through a 

 great range of temperature. The light which passed through 

 the liquid oxygen had a light blue color reiembling that of 

 the sky. 



