372 



NA TURE 



[August 20, 1891 



Inaugural Address by William Huggins, Esq., D.C.L. 

 (OxoN.), LL.D. (Cantab., Edin., et Dubl), Ph.D. 

 (LUGD. Bat.), F.R.S., F.R.A.S., Hon. F.R.S.E., &c., 

 Correspondantde l'Institutde France, President. 



It is now many years since this Association has Hone honour 

 to the science of Astronomy in the selection of its President. 



Since Sir George Airy occupied the chair in 185 1, and the 

 late Lord Wrottesley nine years later, in i860, other sciences 

 have been represented by the distinguished men who have 

 presided over your meetings. 



The very remarkable discoveries in our knowledge of the 

 heavens which have taken place during this period of thirty 

 years — one of amazing and ever-increasing activity in all 

 branches of science — have not passed unnoticed in the addresses 

 of your successive Presidents ; still it seems to me fitting that I 

 should speak to you to-night chiefly of those newer methods of 

 astronomical research which have led to those discoveries, and 

 which have become possible by the introduction since i860 into 

 the observatory of the spectroscope and the modern photographic 

 plate. 



In 1866 I had the honour of bringing before this Association, 

 at one of the evening lectures, an account of the first fruits of 

 the novel and unexpected advances in our knowledge of the 

 celestial bodies which followed rapidly upon Kirchhoff's original 

 work on the solar spectrum and the interpretation of its lines. 



Since that time a great harvest has been gathered in the same 

 field by many reapers. Spectroscopic astronomy has become a 

 distinct and acknowledged branch of the science, possessing a 

 large literature of its own and ob-^ervatories specially devoted to 

 it. The more recent discovery of the gelatine dry plate has 

 given a further great impetus to this modern side of astronomy, 

 and has opened a pathway into the unknown of which even an 

 enthusiast thirty years ago would scarcely have dared to dream. 



In no science, perhaps, does the sober statement of the results 

 which have been achieved appeal so strongly to the imagination, 

 and make so evident the almost boundless powers of the mind of 

 man. By means of its light alone to analyze the chemical 

 nature of a far distant body ; to be able to reason about its 

 present state in relation to the past and future ; to measure 

 within an English mile or less per second the otherwise invisible 

 motion which it may have towards or from us ; to do more, to 

 make even that which is darkness to our eyes light, and from 

 vibrations which our organs of sight are powerless to perceive 

 to evolve a revelation in which we see mirrored some of the 

 stages through which the stars may pass in their slow evolutional 

 progress — surely the record of such achievements, however poor 

 the form of words in which ihey may be described, is worthy to 

 be regarded as the scientific epic of the present century. 



I do not purpose to attempt a survey of the progress of spec- 

 troscopic astronomy from its birth at Heidelberg in 1859, but to 

 point out what we do know at present, as distinguished from 

 what we do not know, of a few only of its more important prob- 

 lems, 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 instrument, and 

 by Prof. Rowland in the construction of concave gratings. 



Lord Rayleigh has shown that theie is not the necessary con- 

 nection, sometimes supposed, between dispersion and resolving 

 power, as besides the prism or grating other details of construc- 

 tion and of adjustment of a spectroscope must be taken into 

 account. 



The resolving power of the prismatic spectroscope is propor- 

 tional to the length of path in the dispersive medium. For the 

 heavy flint glass used in Lord Rayleii^h's experiments, the thick- 

 ness necessary to resolve the sodium lines, came out i'02 cm. 

 If this be taken as a unit, the resolving power of a prism of 

 similar glass will be in the neighbourhood 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 spectroscope 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 



NO. II 38, VOL. 44] 



use ; about 1000 lines being necessary to resolve the sodium 

 lines in the first spectrum. 



As it is often of importance in the record of observations to 

 slate the efiiciency of the spectroscope with which they were 

 made. Prof. Schuster has proposed the use of a unit of purity 

 as well as of resolving power, for the full resolving 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 separa- 

 tion 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 of 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 the 

 solar spectrum, and his scale based upon his original determina- 

 tions of absolute wave-length. It is well known, as was pointed 

 out by Thalen 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 

 adedd 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 MUller and Kempf, which differs 

 very slightly from it, will come to be exclusively 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 determina- 

 tion of relative wave-lengths is simplified to measures of coin 

 cidences of the lines in different spectra by a micrometer. 



The concave grating and its peculiar mounting, in which no 

 lenses or telescope are needed, and in which all the spectra are 

 in focus together, formed a new departure of great importance 

 in the measurement of spedral lines. The valuable method of 

 photographic sensitizers for different parts of the spectrum has 

 enabled Prof. 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 atmosphere. Some recent photographs of the 

 solar spectrum, which [include A, by Mr. George Higgs, are of 

 great technical beauty. 



During the past year the results of three independent re- 

 searches have appeared, in which the special object of the ob- 

 servers has been to distinguish the lines which are due to our 

 atmosphere from those which are truly solar — the maps of M. 

 ThoUon, which, owing to his lamented death just before their 

 final completion, have assumed the character of a memorial of 

 him ; maps by Dr. Becker ; and sets of photographs 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 oxygen. In some 

 recent experiments Olszewski, with a layer of liquid oxygen 

 30 millimetres thick, saw, as well as four other bands, the band 

 coincident with Fraunhofer's A ; a remarkable instance of the 

 persistence of absorption through a great range of temperature. 

 The light which passed through the liquid oxygen had a light 

 blue colour resembling that of the sky. 



Of not less interest are the experiments of Knut Angstrom, 

 which show that the carbonic acid and aqueous vapour of the 

 atmosphere reveal their presence by dark bands in the invisible 

 infra-red region, at the positions of bands of emission of these 

 substances. 



It is now some thirty years since the spectroscope gave us for 

 the first time certain knowledge of the nature of the heavenly 

 bodies, and revealed the fundamental fact that terrestrial matter 



