ON KECENT RESEARCHLS IN THE INFRA-RED SPECTRUM. 471 



with the old method, it will be seen that there are shown in the latter 

 publications about a dozen measured inflections of the energy curve below 

 P'O, and it may be remembered that this curve was obtained only by two 

 years' assiduous labour. 



We have now befoi-e us (fig. 1, Plate II.) three energy curves, obtained 

 by the new method, each exhibiting the whole infra-red spectrum under 

 examination, with about a hundred inflections. These curves are nearly, 

 but not exactly, similar. 



The three were obtained on the same day, each from an entirely inde- 

 pendent observation, so that each has given in a fraction of a day many 

 times the results previously obtained by two years of labour, and, as it will 

 be later shown, has given these results with a notable gain of accuracy. 



But this is not all. These three curves have been taken with a rapid 

 movement of the clockwork and a brief swing of the galvanometer, so as 

 intentionally to suppress all minor inflections and to introduce only the 

 leading features of the spectrum, as shown in eighty or a hundred of the 

 leading inflections (lines) or groups. 



This new bolometric method has, however, as will be shown later, a 

 capacity of resolving these into nearly twenty times that number, the 

 minor inflections having been thus designedly suppressed here, to better 

 show the character and position of the principal ones. All these energy 

 spectra, by the new as by the old method, are, of course, subject to the 

 slight changes due to invisible clouds constantly passing before the sun, 

 which, with the change of the sun's altitude, and of the consequent 

 lengthening path of its rays, prevent any one of them from being exactly 

 like the other ; while, at the same time, everyone here may satisfy him- 

 self, by direct inspection of the i-esults before him, that there is scarcely 

 any single one of their inflections which is not reproduced in the other 

 two, in exactly the same place, though probably not exactly in the same 

 degree ; and when we take difierent spectral traces, made at different 

 hours of the day, and even on different days of the month— traces which 

 are absolutely independent of each other — and superpose them, experi- 

 ence shows that we may expect to see such an agreement as that in the 

 three here chosen at random for illustration, or in the more detailed one, 

 where the relative probable error is less than one second of arc. Three such 

 traces only are here given (to prevent confusion), but if we follow these 

 coincidences through not three, but ten or more plates, we may well judge 

 (since there seems no possibility here of systematic error) that a result 

 which all confirm is reliable, and that, on the other hand, a single inflec- 

 tion on one plate, which the other nine unite in repudiating, is due to 

 some fortuitous cause. 



But there is still a higher certainty to be obtained, by a method 

 independent even of comparison or the exercise of judgment. It is 

 founded on the well-known process of composite-photography, where, 

 in photographing the successive members of an assemblage of persons, 

 having similar general characteristics — as of race, character, or educa- 

 tion, the individual disappears and the normal type alone remains. In 

 order to apply this method to such results as ours, however, another 

 step in the process must be introduced, and this is an interesting one, for 

 the energy curve itself, however valuable, is a comparatively unfamiliar 

 method of showing variations in the energy, which we are all alike used 

 to seeing in the visible spectrum, given by linear representations, and not 

 by a system of inflections. 



