ADDRESS. O 



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 realised 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 farther 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 realise 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 spec- 

 troscopic work continues to be Angstrom's map of the solar spectrum, 

 and his scale based upon his original determinations 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 Com- 

 mittee 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 Piei'ce and Bell, or 

 the Potsdam scale based on original determinations by Miiller and 

 Kempf, which differs very slightly from it, will come to be exclusively 

 adopted. 



The gi-eat 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 determination of relative wave-lengths 

 is simplified to measures of near coincidences 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 togethw, 

 formed a new departure of great importance in the measurement of 

 spectral lines. The valuable 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 atmosphere. Some recent photo- 

 graphs 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 researches have 

 appeared, in which the special object of the observers has been to distin- 

 guish 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 



