96 Frof. Liveing, On the spectra [Dec. 3, 



li N . S='2d.n; 2d dots will be visible and stationary. 



If in place of dots symbols alternately alike are employed thus: — 



/ \ / \ / 



then, by reason of the superposition of adjacent symbols, when 

 SJ^ = 2dn, the appearance is 2d stationary, 



X X X X X 



where d is the wJiole number of symbols employed. Although the 

 difficulty of obtaining a trustworthy standard rotary motion will 

 probably prevent this method from being employed for the absohite 

 measurement of velocities, there is no reason against its adoption 

 for the investigation of the relative speeds of two machines. 



To all cases where it is necessary to study carefully the 

 working of a machine, the method of the Cycloscope can be applied 

 with advantage, while its great elasticity permits it to be adapted 

 to high or low speeds, long or short ranges of velocity, heavy 

 engine machinery or light clockwork with equal facility. 



(2) A communication was made to the Society by 



Prof. Liveing, Note on the spectra of Calcium Fluoride. 



No. 1 in the annexed woodcut is the diagram of the spectrum 

 of fluor spar rendered phosphorescent by heat. I have examined 

 several specimens, and all when they begin to be luminous shew 

 a rather narrow band (a) in the green, and a broad band (/3) 

 extending into the green and blue about equal distances on either 

 side of Fraunhofer's line F. Common nearly white or pale green 

 fluor does not shew any more bands, but as the temperature 

 rises these two bands widen and nearly meet, Derbyshire blue- 

 john shews as the temperature rises a second green band (7) on 

 the yellow side of the first, and then another (^) in the orange. 

 The brighter-coloured green fluor, well known for its phospho- 

 rescence, shews also at the higher temperature two bands (8) 

 and (e) in the purple and violet. In every case the bands (a) 

 and (/3) appear first and are the most persistent. The light is 

 so faint that it is impossible to see any scale or micrometer 

 distinctly at the same time as the bands, but I have taken the 

 position of the bands of the bright green fluor approximately. 

 The position of the -first green band (a) does not appear to be 

 identical in all the specimens, but it is always in the green and 

 on the yellow side of the thallium green line. I have not met 

 with any specimens which shew the spectra described by Kindt 



