PHYSICS — WOOD. 



231 



rescence is stimulated with light of this wave-length the same series is obtained 

 as before, but with the difference that now the lines are of uniform intensity — 

 i. e., the series is not divided into groups by the absence of lines. Stimula- 

 tion with the green cadmium ray gives a fluorescence spectrum consisting of 

 a number of close double lines regularly spaced along the spectrum, and 

 characteristic spectra have been obtained with a large number of monochro- 

 matic stimulations. Thus far results have been obtained with the following 

 sources of light : Vacuum tubes containing helium and hydrogen, quartz arc 

 lamps containing zinc, cadmium, and thallium, and ordinary arcs between 

 copper, lead, silver, bismuth, lithium, and barium electrodes. The stimu- 

 lating radiations are separated and utilized by employing a large bisulphide 

 prism and a long- focus telescope objective. Exposures of ten hours are 

 often necessary, and the metallic arcs require constant attention. The lines 

 are now being measured and the spectra compared, and it is hoped that the 

 results will throw considerable light upon the mechanics of molecular radia- 

 tion. Over 100 lines are present in the fluorescent spectrum excited by the 

 zinc arc. This appears to be the first case in which it has been possible to 

 completely dissect a complicated spectrum into the comparatively simple 

 spectra of which it is composed, by shaking the individual electrons one at 

 a time and noting the effect upon other electrons in the same group. 



cathode; I.UMINESCENCE OF SODIUM VAPOR. 

 This luminescence was first observed by Lewis in a glass vacuum tube, 

 but its spectrum has never been photographed or studied. By employing a 

 steel vacuum tube containing metallic sodium heated red hot and furnished 

 with a large concave electrode of aluminum a blazing disk of yellow light 

 appeared at the point where the cathode rays came to a focus. The spec- 

 trum could be photographed easily through the window of plate glass which 

 closed the end of the tube. It was found to consist of the doublets of the 

 principal and subordinate series, and in addition the complete fluorescent 

 spectrum of the vapor as it appears with white light stimulation. The 

 cathode rays are thus capable of exciting the electrons concerned in the 

 production of the fluorescent spectrum, which remain quiescent when the 

 metal is burnt in the Bunsen flame, electric arc, or introduced into the 

 ordinary vacuum tube. 



THE MAGNETIC ROTATION, TEMPERATURE EMISSION, AND FLUO- 

 RESCENCE SPECTRA OF IODINE. 



The discovery was made last year that the vapors of sodium and iodine, 

 when placed between crossed Nicol prisms in a powerful magnetic field, 

 transmit light which the spectroscope shows as a bright line spectrum. In 

 the case of the vapor of sodium these lines were photographed with the 

 15-foot concave grating, and were found to coincide with certain lines of 

 the fluorescent spectrum. Many of the fluorescent lines are absent or very 



