no 



NA TURE 



\yune II, 1874 



shown that Dr. Draper's idea was heretical, and at the present 

 moment you know it is tlie general opinion of physicists, an 

 opinion founded upon the work which has been done to advance 

 photography, and otlier researches since that time, that the radia- 

 tions which you get from any light source, from the extreme 

 violet to the extreme red, differ only in the rate and in the mag- 

 nitude of the vibrations which are at work, so that I claim for 

 the application of photography to spectroscopy, as a fir^t result, 

 the establishment of a great fact, that the visible, the chemical, 

 and the heat ra)s are really part and parcel of the same thing, 

 that thing being a system of undulations varying in rate and 

 wave-length from one end of the spectrum to the other, whether 

 you consider the visible portion or the invisible rays — those out- 

 side the blue in one case, and outside the red in the other. But 

 this is not all : I claim another tiling for the application of pho- 

 tography to spectroscopy. Sir J. Herschel, so soon as he applied 

 the prism, staled, in a communication to the Royal Society, that 

 it was no longer possible to proceed with that branch of research 

 under the best possible conditions, unless opticians would con- 

 struct lenses which would bring the visible and the chemical rays 

 into absolute coincidence. This is now done by our Rosses and 

 Dallmeyevs in the camera-lenses, and that is the second great 

 feature which I claim for the application of photography to 

 spectroscopy. 



The next step brings us down to the year 1852. In this year 

 a paper i' was communicated to the Royal Society, by Prof. 

 Stokes, who had already announced his discovery of what has 

 since been called " fluorescence ; " "on the long spectrum of the 

 electric light." Prof. Stokes dealt in his firt paper with the 

 "change of refrangibility," or, as Sir William Thomson pro- 

 posed to call it, " degradation of light," by virtue of which, 

 light, which was generally invisible to us, could, under certain 

 circumstances, be made visible. It is no part of my present pur- 

 pose to go into this magnificent paper, one of the crowning 

 glories of the work of this century, at any great length ; but you 

 will see in a moment tbat, if it were a quei^tion of ihedegraiialion 

 of light, then the invisible light to which Prof. Stokes referred as 

 being capable of being rendered visible, must have been light 

 outside the blue end of the spectrum, and not outside the red. 

 Prof. Stokes, in his investigations, in order to get at this invisible 

 light under better conditions, if possible, than those with which 

 he commenced operations, tested the transparency of the sub- 



stances through which the light with which he experimented 

 passed, and the transparency of glass was passed under review by 

 him,* when he found that this invisible light, or whatever it was, 

 could only get through glass with extreme difficulty. Continuing 

 his investigations, he found that quartz on the other hand allowed 

 this invisible li^ht to pass. If you will allow me, I will read an 

 extract from Prof. Stokes's paper of the extremest importance to 

 our subject. After referrmg to these experiments on glass and 

 quartz, he proceeds to say : t — " I have little doubt that the solar 

 spectrum " (which you recollect had already been photi)graphed 

 to a certain extent both by Becqucrel and Draper beyond the 

 visible blue end of the spectrum), "would be prolonged, though 

 to what extent I am unable to say, by using a complete optical 

 train, in every member of which glass v as replaced by quartz, '" 

 He then adds that other substances which suggested themselves 

 to him were not equally good. Then further, that if this invisible 

 light does get through quartz, and does become visible to the 

 eye, it docs not at all follow that it will be capable of being pho- 

 tographed. Because already Prof. Stokes, in order to continue 

 his researches in fluorescence, had been, as it were, driven to 

 photograph some of the results which he had thus obtained. I 

 am sorry to say that, so far as I can find out, none of those pho- 

 togra]>hs have ever been published. 



Before I go further, I think it will be convenient to throw on 

 the screen some photographs of the solar spectrum, showing 

 exactly what I mean by the " invisible rays ;" and you will then 

 see the enormous adva' ce which Prof. Stokes made the moment 

 he introduced his quartz train, and enabled both the eye and the 

 photographer to take advantage of a new^ region of the spectrum 

 in its entirety, in order to investigate it. 



In a no'e to his paper communicated to the Royal .Society, 

 he shows that his anticipations, so far as the eye was concerned, 

 were perfectly justified by the facts. J He says : — " I have since 

 ordered a complete train of quartz, of which a considerable por- 

 tion, comprising, among other things, two very fine prisms, has 

 been already executed for me by Mr. Daiker ; with these I have 

 seen the lines of the solar spectrum to a distance beyond H, 

 more than double that of p. .So that the length of the spectrum, 

 reckoned from II (the outside line in the portion originally visible), 

 was more than double the length of the part previously known 

 from photographic impressions." I will now throw on the screen 

 the spectrum of the extreme part of the visible portion. The eye 



:iir«i:^p^iji«;,(Tr:|iiTTi(|; 



Fig. 2.— The H-!ines in the blue end cl 



generally can see the two dark bands which you see in the middle 

 of the screen called H i and H 2. The least refrangible part of 

 the spectrum lies to the right. When Prof. Stokes, therefore, 

 s'ated that the solar spectrum w.as prolonged, he means that the 

 part of the spectrum visible either to the unassisted eye or on a 

 photogr.nphic plate after impression extends to a certain distance 

 beyond these two dark lines. The part which Prof. Stokes ren- 

 dered visible by means of his quartz train extended a conside- 

 rable distance to the left beyond the part of the spectrum which 

 you now see on the screen. 



So much for the solar spectrum. Now let me carry you on 

 another ten years, to the year 1862. Prof. Stokes, in a paper 

 communicated to the Royal Society in this year, t refers to his 

 former paper, and to what he had been enabled to do by means 

 of it. He states : " A map cf the new lines [the lines thus ob- 

 served by him] was exhibited at an evening lecture before the 

 British Association, at their meeting in Belfast in the autumn of 

 the same year, and I then stated that I conceived we had ob- 

 tained evidence th.at the limit of the solar spectrum in the more 

 refrangible direction had been reached. In fact, tliL- very same 

 arrangement which revealed, by means of fluorescence, the ex- 

 istence of what were evidently rajs of higher refrangibility com- 



• Philoso/'liioil Ti-MSacli.mi:, vol. cl.vii., 1852. 



t On llie long spectrum of tlie electric light. Phil, 'I'r.Tiis , \rA clii p. 



, ^r.iph by the author. 



ing from the electric spark, failed to show anything of the kind 

 when applied to the solar spectrum ;" and then he goes on to 

 say that, in rraking observations by means of the electric spark, 

 he had found that in the case of a spark taken between the poles 

 of an induction coil like this on the table, or between the poles 

 of an electric lamp such as you see there, that the visible spec- 

 trum which was revealeJ and rendered visible to him by means 

 of fluorescence was no less than six or eight time; longer than 

 the whole of the visible part ol the spectrum. That you see, 

 was a revelation of the first order. lie was so astonished at 

 this, that he at first thought there was some mistake. "I could 

 not help suspecting that it was a mistake, arising from the re- 

 flection of stray light." In fact, so astonibhed was he, so many 

 methods did he try in order to break down the impossibility, 11 

 it existed, that he adds, in a subsequent part of the paper, "I 

 tried different methods, without being able to satisfy myself as 

 to the accuracy of the observations, and frequently thought of 

 resorting to photography." 



Prof. Stokes thought of resorting to photography, but at the 

 moment that Pn.f. Stokts was thmking of this, dV. Miller, of 

 King's College (unknown to Prof. Stokes), was not only thinking 

 of resorting to pliotography, but had actu.tlly resorted to it, and 

 was taking photographs of the so-called invisible part of the spec- 

 trum, in which the spectrum in the case of some substances was 

 ' Op. clt. Art. 202. t Art. 204. J Page 559. 



