Page:The American Cyclopædia (1879) Volume XV.djvu/260

 248 SPECTRUM he discovered that similar lines existed in the invisible portion of the spectrum, formed of rays more refrangible than the violet. In 1843 Dr. Draper obtained independently the same results. Mailer, in the sixth edition of his Lehrbuch der Physik, gives a photographic print of the spectral lines extending from a short distance below G to above R. He made this photograph with a prism and lens of quartz. Subsequently Mr. Rutherfurd ob- tained, with tvo prisms of carbon disulphide, a superb photograph embracing lines extend- ing from near & to a considerable distance above the upper of the H lines. This photo- graph has excited universal admiration. It is crowded with lines which are not drawn on the maps of Kirchhoff or of Angstrom. Mas- cart of Paris obtained the Bordin prize of the academy of sciences for his determina- tions of the wave lengths of the visible and ultra violet rays of the spectrum. He used a grating of Nobert to obtain his spectra, and measured the wave lengths of the more re- frangible invisible rays by obtaining photo- graphs on small glass plates placed in the oc- ular E of the spectrometer shown in fig. 4. By this means Mascart measured the wave lengths of these invisible rays with a precision little inferior to that obtained in his measures on the visible rays. He also measured the wave lengths of the light lines given in the spectra of volatilized metals, and found that cadmium gave the most extended spectrum of invisible rays. Mascart observed rays whose wave length was only -00022 of a millimetre. The period of vibration of these shortest waves, compared with the period of the longest visi- ble rays of -00076 of a millimetre, gives about two octaves of the musical scale; that is, the numbers of their vibrations in the same time will be as 1 : 4. But the ratio of the wave length -0019 of a millimetre of the least re- frangible invisible ray of the spectrum to the shortest invisible ultra violet wave of the spec- trum will be as 1 : 8, or as any note is to its upper third octave. The papers of Mascart can be found in the Annales de VEcole nor- male. In 1873 .Dr. Henry Draper of New York published in the "American Journal of Science " a carbon print of a remarkably perfect photograph of a spectrum produced by one of Rutherfurd's diffraction gratings of 6,481 lines to the inch. The negative was photographed on collodion, and the published carbon print has attached to it a scale giv- ing the wave lengths of the rays. This print was obtained by transferring the original neg- ative to a thick plate of glass by a process known as the albertype, and using the glass in a printing press in the same manner as a litho- graphic stone. This print therefore repre- sents the work of the sun itself, and is not ft drawing either made or corrected by hand. The print consists of two portions, the up- per gives all the lines of the spectrum from near G to O, or from wave length -0004350 FIXED LINES. Mascart. Draper. H,... 0008967mm. H' 0008930mm L 0003819 0003821 M 0003729 0003728 N 0003580 '0008580 O 0008440 0003440 p 0008360 Q 0008286

K ;.*..:::::: 0003177 mm. to '0003440 mm. Above this is placed a scale, which is a copy of Angstrom's from just below G to H 2, with the same sized divisions carried out from H 2 to O. The lower part is & magnified portion of the same negative, having Hi and H 3 about its middle, and extending from wave length -0004205 to -0003736 mm. Be- tween wavelengths -0003925 and -0004205 mm., Angstrom's map has 118 lines, while Draper's has 293. We here give a table of the wave lengths of ultra violet rays according to the measurements of Mascart and of H. Draper : Fluorescent Action on the .Spectral Rays. Fluorescence is a property possessed by certain substances of absorbing light composed of rays of a certain wave length, and then emitting this light changed into rays of a longer wave length ; or, what is the same, changed into light of a lower refrangibility. This phenome- non was first observed by Robert Boyle; an account of his experiments may be found in vol. i. of his works (London, 1772). The FIQ. 9. method of examining the spectra of fluorescent bodies is as follows : A porte-lumiere, A, fig. 9, reflects the sun's rays on to a lens at B, which concentrates the light on the fluorescent substances contained in bottles at C. By the revolution of the stand on which these bottles are placed, they can be brought successively before the slit in the collimating telescope of the spectroscope D. Before the solar rays fall on the fluorescent substance they pass through a tank, placed between B and the spectroscope,