﻿508 Pi'of. R. W. Wood on Fluorescence and 



rested eyes. Probably by using a large prism-spectroscope 

 as an illuminator better results can be obtained. What is 

 must desired is a light siren ! 



Analysis of the Complex Spectrum. Stimulation with 

 Mo7iochromatic Light, 



It was found impracticable to use the monochromatic 

 illuminator for the study of the simple spectra which made 

 up the complex spectrum. Even with its slits very narrow, 

 the wave-length range of the emitted light was wide enough 

 to cover several of the absorption-lines of the vapour. The 

 earlier work had shown that the light of the cadmium spark 

 was capable of exciting fluorescence, and experiments were 

 accordingly started with metallic arcs. Just at this time 

 came the very opportune invention of the fused quartz 

 metallic arc-lamps by Stark, working in the Heraeus labor- 

 atory at Hanau. Two of these lamps were immediately 

 ordered, one filled with cadmium, the other with zinc. 

 Their form is shown on Plate XHI. fig. 1. The lamp is 

 kept in communication with a mercury pump during its 

 operation and stands in a dish of water. The cadmium lamp 

 worked well on a circuit of 110 volts direct, but the zinc 

 lamp gave better results on the 220. They are started by a 

 small induction-coil, one terminal of which is connected to 

 the negative pole of the lamp, the other twisted around the 

 quartz U-tube. A blast- lamp is directed against the tube 

 until the portion above the metal electrodes is red-hot, the 

 coil is then started, and the arc usually forms at once. As 

 exposures of eight or ten hours were often necessary, and 

 the lamps have a trick of going out every half-hour or so, an 

 automatic starter was devised, which turned on the coil the 

 moment the lamp w r ent out. As soon as the arc struck 

 again, the coil was stopped, This arrangement is figured on 

 Plate XIII. fig. 2, and consisted of a small electromagnet 

 in circuit with the lamp, which pulled a steel spring away 

 from a brass screw as long as the lamp burnt. The spring 

 and screw were inserted in the primary circuit of the coil. 



The cadmium lamp burns with a greenish-blue light of 

 dazzling brilliancy, the zinc lamp with a curious purple light, 

 which causes all the woodwork in the room to appear blood- 

 red, while most other objects appear bluish white or purple. 

 Both lamps excite a brilliant fluorescence of the sodium 

 vapour when their images are thrown upon the oval aperture 

 of the retort. In this case the fluorescence is excited by 

 several different radiations. Various devices were used for 

 picking out one line at a time. The cadmium radiations 



