166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



is still in progress, and many remarkable observations have been 

 made, each one leaving us more in the dark than before. As an illus- 

 tration I may mention a circumstance discovered by Dr. Hemsalech 

 and myself last winter in Paris. We found that if a jet of air was 

 blown through the squirt of light, the luminosity was destroyed in the 

 region traversed by the moving current of air, but was of undimin- 

 ished intensity both above and below this region. This makes it 

 seem as if the emanation which comes from the spark, and which 

 causes the luminosity of the air, must act for a brief time upon the 

 air in order to cause the luminosity. It also shows that the emana- 

 tion, whatever may be its nature, is not swept aside by the air current. 

 We have also found that other gases become luminous when sub- 

 jected to the spark emanations, the spectrum in each case being 

 different and peculiar to the gas used, electrolytic hydrogen, for 

 example, giving a strong luminosity. 



It is thus apparent that by employing this "photographic eye" of 

 quartz many new phenomena may be brought to light which have 

 previously hidden themselves behind the limitations of the human 

 eye. A study of the absorption by the candle-flame of ultra-violet 

 has also been made. In this case the light emitted by the candle 

 falls out of the problem, for its flame emits little or no ultra-violet. 

 I can show you a photograph of the shadow cast by a flame of this 

 description, and you will observe that the shadow is blackest at the 

 point where the flame is brightest, that is, at the point where the 

 minute carbon particles, which, by their incandescence, cause the 

 luminosity, are being set free from the hydrocarbon vapor. 



There are other questions which can doubtless be investigated to 

 advantage by means of ultra-violet photography. It is well known, 

 for example, that the amount of ultra-violet light emitted by a body 

 increases with the temperature. By photographing groups of stars 

 through the quartz silver filter, and comparing the photometric 

 intensities of the images obtained in this way with the intensities 

 as shown on a plate made by means of yellow light, valuable data 

 might be obtained. This method is merely an extension of one 

 already in use at the Harvard Observatory. 



