A N:o 1) On Light-absorption in a Pleochroitic Uniaxial Crystal. 3 



crystal. The Nicol is first adjusted so as to completely 

 extinguish the last named beam. Let f^ represent the angle 

 read at the time. Thereupon the Nicol must be turned until 

 the light intcnsities of the two images are equal. Let u^ 

 stånd for the angle now read. The relation between the two 

 intensities of the beams entering the microscope M, is evi- 

 dently given by the expression: 



tg'(y2-^i) =tg^v- 



The absorption of the extraordinary (t) rays can therefore 

 be determined in the above indicated manner. 



Then the crystal is transferred to the object table G, 

 and the oscillatory directions in the crystal must exactly 

 correspond with the oscillatory directions in the spar. This 

 adjustment of the crystal can be easily effected by employing 

 the Nicol, after which the relation between the intensities of 

 the It) (ordinary) rays and e (extraordinary) rays can at once 

 be determined. The angle of the revolution of the Nicol 

 corresponding to the previously mentioned angle v, is here 

 indicated by V. 



Owing to the ray's passage through the prism in spectro- 

 scope C and the spar I, and owing to its reflection in mirror 

 E, the intensities of the two beams issuing from the spar I, 

 are a priori unequal. The relation between these intensities 

 was found to be C =1,628. 



From the experiment it appears that the reflection from 

 the surfaces of the turmaline plates may be disregarded 

 because in the case of the strongly absorbent medium in 

 question, any error due to the cause mentioned lies below 

 the limit of experimental errors. The absorption may there- 

 fore be represented by the formulae: 



