462 CARNEGIE INSTITUTION OF WASHINGTON. 



mined as accurately from 1 day of corrected observations as it could 

 formerly be determined from 4 days of uncorrected observations. 



It is important to determine the fluctuations in the elevation of the 

 mean surface of each lake, because such fluctuations are the measure 

 of the variations in the total water-content of the lake. 



The application of the corrections for barometric effects and wind 

 effects give a sound and accurate basis for the proposed investigation 

 of evaporation. 



Nichols, E. L., Cornell Univcrsit}', Ithaca, New York. Report on studies in 

 luminescence. (For previous reports, see Year Books Nos. 4-19.) 



On the subject of the excitation of fluorescence by the hydrogen 

 flame, to which reference was made in the Year Book for 1920, four 

 papers have been published, as follows : 



Luminescence at hic^h temperatures: E. L. Nichols and D. T. Wilber. 



Flame excitation of luminescence: E. L. Nichols and D. T. Wilber. 



The luminescence of certain oxides sublimed in the electric arc: E. L. Nichols and D. T. 



Wilber. 

 The spectral structure of the luminescence excited by the hydrogen flame: H. L. Howes. 



Since the completion of these investigations our studies have been 

 directed to another phenomenon, described as follows. 



The Blue Glow. 



This is a luminescent effect which is superimposed upon the ordinary 

 temperature radiation of certain oxides. In so far as this effect has 

 been noticed hitherto, and it is too obvious to have escaped attention 

 altogether, it has been designated as "selective radiation," and having 

 been given a name it has been ignored. That the blue glow is a type of 

 luminescence will appear clearly from the following characteristics 

 already established in the course of the investigations now in progress 

 by Professor Howes and the writer : 



(1) It is characteristic of two groups of oxides: (a) of the oxides of calcium, 

 magnesium, beryllium, zirconium, silicon, and aluminum; (6) in somewhat 

 modified form, of such of the rare earths as have been tested, i. e., the oxides 

 of samarium, gadolinium, prsesodjmiium, neodymium, erbium, and cerium. 



(2) In group (a) the radiation in question is chiefly that lying within a 

 broad blue band in the spectrum. 



(3) The effect is most strikingly obtained by heating the oxide directly in 

 the oxy-hydrogen flame, but it occurs when other means of heating are used. 



(4) There is a definite temperature range within which the blue glow 

 begins, reaches its maximum, and disappears. This differs with the indi- 

 vidual oxides, but in all cases thus far examined the limits are found between 

 a dufl red heat and 1200° C. 



(5) The effect accompanies and is probably intimately related to the trans- 

 formation of these oxides from transparency to opacity and their change from 

 dielectrics to conductors of electricity. 



(6) It is subject to fatigue. 



(7) It is profoundly modified by previous heat-treatment of the oxide. 



(8) It occurs in oxides which are non-luminescent under photo-excitation 

 or the action of X-rays, but which are capable of kathodo excitation. 



