﻿506 Prof. R. W. Wood on Fluorescence and 



large supply of metal to be kept at the centre of the tube. 

 The tubes used in the earlier work required re-charging after 

 two hours' continuous operation, while the retort-tube could 

 be operated for several hundred hours on a single charge. 



The tube was exhausted with a Fleuss pump and heated 

 at the centre with a large burner, the ends being kept cool 

 by jackets of absorbent cotton which dipped into pails of 

 water. 



The illuminating beam of either white or monochromatic 

 light was focussed jur-t within one of the oval apertures of 

 the retort, falling upon the opposite wall a little to one side 

 of the other aperture. By covering the further end of the 

 tube with a black cloth, the fluorescent spot showed against 

 the dead black background of the second oval aperture, and 

 its spectrum was therefore uncontaminated with the exciting- 

 radiations. 



A large three-prism spectrograph was constructed for 

 photographing the spectra. The prisms were of clear dense 

 flint four inches in height, and the focal length of the lenses 

 thirty-six inches. 



Since only lenses such as are used for telescopes were 

 available, the spectrum lines are not so sharp as one would 

 wish, except near the axis of collimation. By adjusting 

 things so that the centre of the fluorescent spectrum fell at 

 this point, the definition was pretty fair throughout its extent, 

 and wave-lengths could be determined with an error not 

 greater than one or two Angstrom units. 



The photographs of the complex spectrum of the fluorescence 

 excited by sunlight, obtained with this instrument PI. XIV, 

 fig. 1, showed peculiarities which made it appear of the utmost 

 importance to study the spectrum under higher dispersion. 

 The green fluorescent spot had, after repeated improvements 

 in the apparatus, attained such brilliancy that I felt sure that 

 records could be obtained with the twelve-foot concave 

 grating. An all-day exposure was found to be sufficient, the 

 resulting spectrogram, with the iron comparison lines and 

 the wave-length scale, being reproduced on PL XI. fig. 2. 

 The scale was printed separately, and slight errors occur, 

 due to stretching and shrinking of the prints. They are not 

 greater in any case than 1*5 A.E. This plate shows the 

 minute structure of the complex spectrum, and enables us to 

 measure the wave-lengths of the bright lines with far greater 

 accuracy than could be done with the plates made with the 

 prism-spectrograph. As much of the detail is lost during 

 the process of reproduction, I have prepared a very careful 



