244 M. E. Pringsheim on a Measurement of Wave-lengths 



red rays; consequently its approximate value is 



\= 0*00152 millim. 



In order to determine the breadth of the ineffective strip, 

 the wave-length of the first rays again, effective was measured, 

 with the following result: — 





a. 



b. 



c. 



.\. 



Aug. 7, with iodine solution. 

 ,, „ ebonite 



165 3 32-0 



166 25 14-3 



183 31 21-4 

 182 41 93 



153 34 21-5 

 153 13 51-5 



millim. 

 0-0013908 



0-0013864 





The insensitive band extended therefore, on the employ- 

 ment of the iodine solution, from X = 0*001 3658 to \ = 0-0013908 

 millim., and, with the ebonite plate, from \= 0*0013834 to 

 X= 0*0013864 millim. On the reason of this insensitiveness 

 nothing exact can at present be stated. There are, how- 

 ever, three possibilities : either a group of Fraunhofer's lines 

 exist at this place; or both the plate of ebonite and the 

 iodine solution are adiathermanous for rays of this particular 

 refrangibility; or the lampblack does not absorb them. The 

 reason that the band is much broader when iodine solution 

 is employed than with ebonite would, with all three ex- 

 planations, be found in the circumstance that the iodine 

 solution is, on the whole, less transparent at this part of 

 the spectrum than the ebonite. 



Y. Practical and Theoretical Limits. 



In our investigation we met with the practical difficulty 

 that the dark rays of the spectrum of the first order coincided 

 with those of the second. Although we have not at present 

 the means of separating these two kinds of rays, it is very 

 probable that, by careful investigation of the diathermancy of 

 various materials, a substance will be discovered impervious 

 to the first ultra-red rays and transmitting only the rays from 

 a certain great wave-length onwards. 



Now our arrangement above described is very convenient 

 for the investigation of the diathermancy of any substance, since 

 one has only to place the substance in question before the 

 torsion-apparatus to ascertain with facility, on rotating the 

 grating, all the parts of the spectrum at which the substance 

 transmits the rays. Nay, even a quantitative determination of 

 the diathermancy could be deduced, at least approximately, 

 from the strength of the deflection produced. 



