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XL. Intelligence and Miscellaneous Articles. 



THE PHOTANTHISTAN : A NEW INSTRUMENT FOR THE COM- 

 PARISON OF LUMINOUS INTENSITIES AND ABSORPTION CO- 

 EFFICIENTS. BY J. J. TAUDIN CHABOT. 



A KNOWN method of showing the sensitiveness to light of: 

 selenium consists in illuminating a so-called selenium cell 

 intermittently by means of a rotating opaque disk provided with 

 slits, light from a suitable source alternately passing through the 

 slits in the disk and being cut off. The conductivity of the 

 selenium is found to undergo fluctuations, which — if a constant 

 potential difference be maintained across the terminals of the cell — 

 result in fluctuations in the current. 



In the case just considered, the selenium is exposed alternately 

 to the luminous radiation from a source of given intensity, and a 

 source of zero intensity. This consideration suggests the construc- 

 tion of the very useful instrument which forms the subject of this 

 article. We may, instead of using a second source of zero 

 intensity, allow two sources, the intensity of neither of Avhich is 

 zero, to illuminate the selenium cell. In general, fluctuations will 

 in this case also take place in the current traversing the selenium 

 cell, with the exception of the special case in which the illumina- 

 tions due to the two sources are equal. The selenium then 

 behaves as if it were subject to a constant illumination — it has a 

 corresponding constant resistance. This special case forms the 

 principle of action of the photanthistan. In this instrument, the 

 luminous radiations from the two sources under comparison are, 

 by the use of a slotted rotating disk, allowed to fall alternately on 

 the selenium cell. An Einthoven string galvanometer in the 

 circuit of the selenium cell indicates by its vibrations when the 

 illuminations are unequal ; these vibrations give place to a steady 

 deflexion when the distances of the sources are to one another in 

 the ratio of the square roots of their intensities — assuming equal 

 absorptions of all the media traversed. Exceptional sensitiveness 

 is attained by making the frequency of the alternate illuminations 

 correspond to the natural frequency of vibration of the galvano- 

 meter, resonance conditions being thereby obtained, and very 

 slight differences in the illuminations being sufficient to call into 

 play vibrations of the galvanometer. 



Thus it is possible by the simplest geometro-mechanical method 

 to compare the luminous intensities of sources of equal and 

 different wave-lengths, and the absorption coefficients of media 

 interposed in the path of the radiations. 



Degerloch (Wtthg.), May 28, 1£07. 



