2 Jarl A. Wasastjerna. (LXIV 



chromatic beam, which, reflected by the mirror E, is further 

 regulated by slit F, the aperture of which can be regulated 

 with precision by the aid of two micrometer screws, placed 

 perpendicularly one to another and not visible in the drawing. 



That portion of the apparatus indicated by letters E — O, 

 must be imagined as turned round through an angle of 90 

 degrees, and thus brought into a vertical position. 



G, H, K and L indicate glass plates ground perfectly 

 plane, and letter I indicates a very perfect piece of Iceland 

 spar ground and polished, which may be turned round an 

 imaginary axis lying in the longitudinal direction of the 

 apparatus. The two parallel beams passing through the Ice- 

 land spar I, are received by the object glass of the micro- 

 scope M, the eye-piece of which is provided with a revolving 

 Nicol N. The position of the Nicol is read dired in whole 

 degrees, and the Vernia O indicates multiples of 5'. 



For obtaining mono chromatic light of a given wave- 

 length, we must proceed as follows: The lens-system B is 

 completed with slits by means of which the axis of the lens- 

 system and the image of the are A lying in the lens-system 

 are defined with precision. The arc-light is replaced by a 

 sodium lamp, after which the slits D and F and the mirror 

 E are placed in a position to illuminate the field of vision 

 of the microscope. When the sodium lamp is again replaced 

 by the are A the wave-length of the light falling into the 

 system F — O, can be modified as desired, by means of a 

 simple manipulation. This is done by the aid of a micrometer- 

 screw provided with a scale which directly indicates the 

 wave-length and which micrometer-screw turns the prism 

 in the spectroscope C. 



The experiment itself consists in first placing the turm- 

 aline plate on the object table K in such a manner that 

 only the one beam passes through the plate, and the 

 Iceland spar is turned so that the principal section, per- 

 pendicular to the surface of the turmalin e plate coincides 

 with the plane of oscillation of the beam which can be verified 

 without difficulty by the aid of the Nicol N. The other beam 

 falls direct on the object glass, without passing through the 



