prising, among other things, two very fine prisms, has been already executed for me by Mr. Darker; 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 spectrum, reckoned from H (the outside line in the portion originally visible), was more than double the length of the part previously known from photographic impressions. The eye generally can see the two dark FIG. 29. The H-lines in the blue end of the solar spectrum, from a photograph by the author. bands represented in the middle of Fig. 29, and lettered HI and H 2. The least refrangible part of the spectrum lies to the right. When Prof. Stokes therefore stated that the solar spectrum was prolonged, he meant that the part of the spectrum visible either to the unassisted eye or on a photographic plate after impression, extends to a certain distance to the left of these two dark lines. The part which Prof. Stokes rendered visible by means of his quartz train extended a considerable distance to the left, beyond the part of the spectrum represented in the figure. So much for the solar spectrum. Prof. Stokes, in A map a paper communicated to the Royal Society in 1862,* refers to his former paper, and to what he had been enabled to do by means of it. He states: of the new lines" [the lines thus observed by him] "was exhibited in 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 obtained evidence that the limit of the solar spectrum in the more refrangible direction had been reached. In fact, the very same arrangement which revealed, by means of fluorescence, the existence of what were evidently rays of higher refrangibility coming 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 making 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, or between the poles of an electric lamp, that the visible spectrum which was revealed and rendered visible by means of fluorescence was no less than six or eight times longer than the whole of the visible part of the spectrum. This was a revelation of the first order. He was so astonished at it, that he at first thought there was some mistake. "I could not help suspecting that it was a mistake, arising from the reflection of stray light." In fact, so astonished was he, so many methods did he try in order to break down the impossibility, if it existed, that he adds, in a subsequent part of the paper, "I tried different methods, * On the long spectrum of the electric light. Phil. Trans. vol. clii. p. 599. without being able to satisfy myself as to the accuracy of the observations, and frequently thought of resorting to photography." § 4. Dr. Miller's Work. While Prof. Stokes was thinking of resorting to photography, Dr. Miller, of King's College, was not only thinking of it but had actually resorted to it, and was taking photographs of the so-called invisible part of the spectrum, in which the spectrum in the case of some substances was five or six times, and in the case of silver one might say almost seven times, as long as the spectrum ordinarily visible through glass prisms. Prof. Miller went very nearly over the same ground that Prof. Stokes had done before him. He investigated the transparency of quartz, and came to the conclusion that quartz is almost the only substance that can be employed. He also gives for the first time a detailed account of the way in which such work is done. We have first a spark from an induction coil, between poles composed of the metals the spectra of which Dr. Miller wished to examine. A quartz lens to throw the image of these poles on to the photographic plate, prisms of quartz, and a camera; so that he had, first of all, a light source by which he got an intense illumination, then a quartz train as it is called, and then simply the photographic plate. Having therefore an entire absence of non-transparent glass, Prof Miller was * Vol. cit. p. 801. delighted to find that, on taking the spark in this way, between electrodes of different substances, he not only photographed part of what could be seen, namely, a spectrum ranging from green to blue, but one extending as a rule six times the length of the visible spectrum beyond the blue; although, in some cases, it is true it was 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 from the line A. FIG. 30.-Dr. Miller's arrangements,-s, slit; 7, quartz lens ; c, camera;, quartz prism; t, collimator; e. spark. In this paper of Dr. Miller's we have the germ of all the applications of photography to spectroscopic inquiry which have 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 radiation of different vapours, and give photographs for the first time of the bright lines of a very large number of chemical elements, but he went further than this, and dealt with the absorption of many substances. |