226 Intelligence and Miscellaneous Articles. 



The lines marked with an asterisk were observed in 1876 by my 

 father, who determined the wave-length 0*001220 by observation 

 of the interference-bands which are produced in the spectrum by 

 previous reflection of the luminous rays upon a thin film of air. 



The last two numbers of the preceding Table are obtained by 

 carefully tracing a curve giving the wave-lengths for the various 

 positions in the spectrum and prolonging it beyond the line A/", 

 which answers to the last number found by experiment ; and of 

 those two numbers the last presents the curious coincidence that it 

 is very nearly the w r ave-length of the cold band determined by M. 

 Eizeau. 



The new points resulting from the present researches are, besides 

 the determination of new lines of the solar spectrum and their 

 wave-lengths, the observation in the infra-red portion of maxima 

 and minima of extinction proper to various phosphorescent sub- 

 stances, manifested by various luminous sources and analogous to 

 the phosphorogenic maxima and minima of the other end of the 

 spectrum. — Comptes Renclus cle VAcademie des Sciences, Jan. 8, 1883, 

 t. xcvi. pp. 121-124. 



ON THE MEASUREMENT OF THE PHOTOMETRIC INTENSITY OF THE 

 SPECTRAL LINES OF HYDROGEN. BY H. LAGARDE. 



The spectrum of a gas, under determined conditions of tempera- 

 ture and pressure, is not completely defined by the wave-lengths 

 of the various lines which compose it. When the pressure and 

 the calorific energy of the discharge are varied, the intensity of 

 those lines is modified according to an unknown law ; it may even 

 become nil for one or more particular lines, under certain condi- 

 tions ; while other lines may become visible for particular values of 

 the temperature and pressure. These variations of intensity there- 

 fore, in each circumstance of the experiment, change the phy- 

 siognomy of the spectrum, which will not be defined unless the 

 intensities of the lines composing it be given. It is to the study 

 of the measurement of these intensities that I have applied myself. 



In absolute value, the radiant energy of a vibration of a deter- 

 mined wave-length ought to be expressed in thermal or mechanical 

 units ; but the feebleness of gas-spectra forbids any direct attempt 

 in that way, and necessitates the use of a photometric comparison. 



The employment of a spectrophotometer disposed so as to give 

 precise and comparable valuations is of course imposed, as M. Crova 

 has shown*, in determinations of this sort. The capillary portion 

 of the spectrum-tube being placed opposite half the slit of the 

 instrument and at a constant distance, the light from a lamp, 

 having traversed a system of two nicols, one movable upon a gra- 

 duated circle, is received laterally upon a prism with double total 

 reflection, which covers the other half of the slit. If the slit be 

 opened a little, the lines of the spectrum take sufficient breadth to 

 till the ocular slit and are in immediate contact with the portion of 

 the spectrum of the lamp possessing the same wave-length ; the 

 rotation of the nicol is measured which gives equality of intensity. 



* Comptes Rendus, 1881, t. xciii. p. 512; Ann. de Chim. et de Phys. [5] 

 xxii. p. 513. 



