30 FLUORESCENCE OF THE URANYL SALTS. 



this sort in case the light emitted were strictly proportional to the 

 coefficient of absorption ; but if the fluorescence band and the absorp- 

 tion band do not exactly coincide in position or in form, such a dis- 

 placement is to be expected. 



In order to avoid the necessity of changing the adjustment of the 

 spectrophotometer, or the position of the substance, between measure- 

 ments a thin layer of the uranyl potassium sulphate was in some cases 

 mounted permanently in front of the slit. To locate the absorption 

 bands the slit was illuminated, through the specimen, with light from an 

 acetylene flame. To observe the luminescence bands a piece of blue 

 glass was placed in front of the flame, so as to cut off the rays having 

 the same wave-length as the bands, while permitting the exciting rays 

 to pass ; or in some cases the acetylene flame was replaced by a mercury 

 arc. To guard against the presence of fluorescence in measurements 

 of absorption a green glass was sometimes used. 



With the relatively thick specimen first used the absorption was so 

 great that the band at 4,760 could not be observed. The band at 4,920 

 was well defined, however, and could be accurately located. If the 

 eyepiece pointer was set at the crest of the absorption band and the 

 source of light then changed so as to bring out the fluorescence band, 

 the latter was seen to be very obviously displaced toward the red. 

 Photographs of the absorption and fluorescence spectra taken on the 

 same plate also showed the relative displacement of the two bands very 

 clearly. The wave-length of the fluorescence band as measured under 

 these conditions was not the same, however, as that previously deter- 

 mined, and the whole appearance of the band was different from what 

 had been observed when looking at the front surface of the luminescent 

 substance. 



More definite conditions for observing the absorption band were 

 obtained by using nearly monochromatic light for transmission meas- 

 urements. The spectrum of a Nernst glower was formed by a large 

 spectrometer and a small region of the spectrum was isolated by means 

 of a suitable screen containing a slit. The light coming through this 

 slit, after passing through the specimen to be studied, fell upon the slit 

 of the spectrophotometer. By suitable adjustment the center of the 

 band of transmitted light could be made practically coincident with 

 the center of the absorption band and the latter could be located with 

 considerable accuracy. Under these circumstances the transmitted 

 light contained no rays capable of exciting any observable fluorescence, 

 so that we may look upon the determinations of absorption by this 

 method as uninfluenced by errors due to the presence of luminescence. 



Using a relatively thick layer, the absorption band was located at 

 4,919, while the crest of the fluorescence band (observed by transmis- 

 sion) lay at 4,974. An excessively thin layer, formed by depositing 

 the salt from a solution, or suspension, in alcohol, gave a fluorescence 



