Dr J. H. Gladstone on Fluorescence. 83 



an hour after observing the sun. The Greenwich mean time 

 of the sun's transit across the theodolite wires was thus oh- 

 tained, — the correction for the rate of the chronometer in the 

 short interval between the observations of the sun and the 

 time-ball never exceeding S *1. The sun's hour-angle, at 

 the instant of his transit across the middle wire of the theodo- 

 lite, was then easily found from the known longitude of the 

 station ; and the deviation of the plane of the instrument from 

 the meridian was calculated with sufficient accuracy by the 

 formula 



Sun's hour angle x cos. sun's declination x cosec. sun's zen. dist. 



In this manner, on various days, I obtained seven observa- 

 tions for the azimuth of the true north, the greatest difference 

 between any single observation and the mean of the whole 

 being 30". 



I have ventured to give this brief description of the process 

 by which the variation of the needle given above was ascer- 

 tained, not on account of any novelty it possesses, but merely 

 to enable the reader to judge what degree of reliance is to be 

 placed in the result. 



Notes on some Substances which exhibit the phenomena of 

 Fluorescence. By Dr J. H. Gladstone, in a Letter to 

 Dr Anderson. 



To Professor Anderson, M.D., F.R.S.E. 



My dear Sir,— When I read my short communication 

 " On the Fluorescence exhibited by certain Iron and Platinum 

 Salts," at the Liverpool meeting of the British Association, 

 you kindly offered to publish the results of my observations 

 in the Philosophical Journal. I felt then that my experi- 

 ments were very incomplete ; but since that time I have re- 

 peated most of them, examining the blue appearance more 

 critically ; and I now transmit to you a fuller description of 

 the phenomena. Will you allow me also to add an account 

 of a few other substances that exhibit fluorescence, but which 

 have come under my notice more recently. 



f2 



