jjlne II, 1874J 



NA TURE 



five or six times, and in the case of silver 

 one might say almost seven times, as long 

 as the spectrum ordinarily visible through 

 glassprisms. Prof. Miller goes very nearly 

 over the same ground that I'rof Stokes had 

 done before him. He also investig.ated the 

 transparency of quartz, and comes to tlie 

 conclusion that quartz is almost the only 

 substance that can be employed. Prof. 

 Miller, in this paper, which you will find in 

 the Philosophical Ti'aiisactions* also gives 

 for the first time a detailed account of the 

 way in which such work is done. Permit 

 me to give you a rough notion of this me- 

 thod of work. We have liere a spark from 

 an induction coil, exactly such a spark as 

 Dr. Miller wished to examine. He had a 

 spectroscojie something like thi? on the 

 table, with two important differences. The 

 first important difference was that instead 

 of having two glass prisms he h.ad prisms 

 of quartz ; and again, instead of having 

 an observing telescope adapted for use by 

 the eye, he inserted a camera, or what was 

 to all intents and purposes a camera, in the 

 same place. So that he had, first of all, a 

 light source by which you get an intense illu- 

 mination, due, as is generally imagined, to 

 the extremely liigh temperature of the spark. 

 Then you have a quartz lens, and quartz 

 prisms, and then simply the pliotographic 

 plate. Having therefore an entire absence 

 of the non-transparency of glass. Prof Miller 

 was delighted to find that, on taking this 

 spark in this way, between electrodes of 

 different substances, he not only photo- 

 graphed what could be seen, namely, a 

 spectrum ranging Irom red to blue, but one 

 extending as a rule six times the length of 

 the visible spectrum beyond the blue ; al- 

 though, in some cases, it is true it is only 

 four times as long on the more refrangible 

 side of H, as H is from the red end of the 

 spectrum, that is to say the line which is 

 generally called A. In this paper of llr. 

 Miller's we have the germ of all the appli- 

 cations of photography to spectroscopic in- 

 quiry which hare been carried on since ; 

 and I am sorry to say that altogether too 

 little has been carried on. Not only did 

 Dr. Miller investigate in this way the radia- 

 tion of different vapours, and give photo- 

 graphs for the first time of the bright 

 lines of a very large number of chemical 

 substances, but he went further than this, 

 and dealt with the absorption of different 

 substances. 



He commences his paper with the absorp- 

 tion of chemical rays by transmis.sion through 

 different media, — through solids (iranspa- 

 rent, of course), through liquids, and 

 through gases and vapours, the only altera- 

 tion he made in his general mode of expe- 

 rimentation being that in the case of the 

 absorption of gases and vapours he )ilaced 

 the instrument farther from the light source, 

 and in the path of the ray inserted a tube 

 containing the gas or vapour to be experi- 

 mented with, as I am doing now, so that 

 the li^ht which passed from the spark to 

 the telescope was compelled to traverse a 

 thickness of vapour according to the length 

 of the tube employed. In that way he 

 not only determined the absorption of 

 equal lengths of dificrcnt vapours amongst 

 themselves, but the absorption of different 

 lengths of the same vapour; his paper is ^UnCO 

 thus one of llie most important contribu- '* 



tions to spectroscopic knowledge th»t I 

 am acquainted with, and I hold that the 

 chief importance of it is the application of 

 ' Vol. cit. p. 801. 



T/iiuV 



TtinCl 



Figs. 3 and 4*.— Coi- 



i of Dr. Miller's m , 



liouii g ihe knglli oi the 1 



been obligingly placed st my disposal by Messrs. Longman 



J. N. L. 



