474 REPOKT — ^1894. 



communication to this Association, but here resolved into thirty or more 

 subordinate lines. This illustration includes a part of the new region 

 discovered on Mount Whitney in 1881, and in the small portion here 

 exhibited it may be seen that about 200 lines are discriminated. 



I am now trying to bring what may be called the first stage of the 

 long labour, a portion of which is here described, to a close, this first 

 stage consisting chiefly in the disco^•ery of and mapping the relative posi- 

 tions of new spectral lines. 



I will only refer to what it seems to me the second part of this work 

 is likely to be, and to the diSerent kind of interest which may not 

 improbably belong to it, from that which belongs to this the first. 



"We are thus far in the position of early students of the visible 

 spectrum, who simply drew the lines they saw without inquiring into 

 their meaning. Nevertheless, to have discovered and mapped a great 

 number of these lines is only a beginning, for their real ^alue lies in their 

 interpretation, and this is still chiefly to come. As to the possible 

 importance of this interpretation, it is not enough to remind ourselves 

 tliat three-quarters of the whole energy of the sun exists here and not in 

 tlie upper spectrum, We must remember also that while, as a rule, in the 

 upper and visible spectrum a great proportion of the lines are caused by 

 absorption in the solar atmosphere, and a perhaps smaller portion by 

 telluric absorption ; here, on the contrary, we are led by everything wi; 

 already know to expect that the great telluric absorptions on which 

 meteorological predictions and other inniiediately practical interests 

 depend may be expected to be found, and it is on the comparison of 

 these energy curves taken at different periods of the year and at different 

 altitudes of the sun, that those who are engaged in the work see good 

 cause to hope for important results in the future. 



Before I conclude let me present a collective view of (fig. 4, Plate IV.) 

 the field in which work has been going on in these later years at the 

 Smithsonian Observatory on the same scale with the visible spectrum ; 

 I say ' on the same scale,' meaning not on a wave-length scale which 

 expands the invisible at the expense of the visible, and not on a prismatic 

 scale alone which expands the visible at the expense of the invisible, noi' 

 even on such a logarithmic one as that proposed by Lord Rayleigh, but on 

 a conventional scale, which I will ask you to tolerate, as- it is simply 

 meant to show the actual extent and importance of the region covered 

 here, as compai'ed with that known to Newton. In this illustration, with 

 which I close my remarks, the mean dispersion throughout the invisible 

 rock salt spectrum, as far as 4" is taken as the standard, and both spectra 

 are laid out on that common scale. On the left is the A'isible spectrum 

 known to Newton : next this is the region known through photography, 

 now extending a little beyond the band per which marks what, at the time 

 these researches were commenced, was considei-ed by the then most 

 "distinguished investigator in the infra-ied, the end of the heat spectrum. 

 Beyond, and on the right, is a part of the new regions of the spectrum 

 developed by the bolometer, and of which charts may be shortly expected 

 on the scale of which a specimen in detail has just been shown. 



I cannot close this statement without expressing the gratification with 

 which I have laid it before the same body that listened to that made on 

 the same subject twelve years ago, or my sense of my good fortune in 

 doing so before an audience in which I recognise many of the same 

 eminent men who so kindly received that first presentation of these 

 researches. 



