and prime Azimuth of Fraunhofer's Lines. 323 



trum with Fraunhofer's lines not objectively, but subjectively, 

 and permits the prime angle of incidence and prime azimuth to 

 be determined with any accuracy desired, as soon as reflecting 

 surfaces generally can be produced of homogeneous quality. 



Through a collimator with a vertical slit and an achromatic 

 lens, a pencil of parallel rays falls upon a polarizing prism stand- 

 ing in the azimuth 45°, and then upon a horizontal astrono- 

 mical auxiliary telescope, the cross-threads of which can be illu- 

 minated by a plane glass (between the eyepiece and the cross- 

 threads) inclined 45° to the axis of the telescope. 



Across the slit of the collimator runs a thin wire, to which 

 the cross-thread of the telescope is adjusted. It lies on the axis 

 of the collimator when its position is unaltered on turning the 

 latter 180°. 



Between the collimator and the telescope is a large smoothed 

 cast-iron frame, on which an iron sledge with a horizontal circle 

 can be moved parallel to itself. A little table in the centre of 

 the circle, and the circle itself, are to be moved each by three 

 adjusting-screws, and the rotation of the table to be measured 

 by two diametral verniers. On the table a plane-parallel glass 

 is fixed, so that the image of the cross-threads of the telescope 

 is reflected towards the latter. When the plane glass is rotated 

 180° the cross-threads and their image again coincide, if the 

 axis of rotation of the circle is normal to that of the collimator. 

 The normal of the plane glass and the axis of the collimator then 

 coincide ; the angle of incidence is 0°. 



The azimuth 90° of the polarizing Nicol is determined thus : — 

 The slit of the collimator is illuminated with daylight or with a 

 sodium-flame, the angle of incidence made nearly equal to the 

 angle of polarization, and the glass plate and the polarizing 

 Nicol rotated until the image of the slit, observed with the naked 

 eye or a telescope, shows a minimum intensity of light (0). 



The analyzing Nicol, fixed on a vertical circle, stood at 0°when, 

 after the removal of the plane glass, the light issuing from the 

 strongly illuminated slit, and polarized by the Nicol in the azi- 

 muth 0°, was completely extinguished. 



The vertical circle with the analyzing Nicol could either be 

 moved parallel to itself on a horizontal plane table-plate, or it 

 was fastened in front of the objective of a telescope which was 

 directed to the slit of the collimator and could be moved parallel 

 to itself on a cast-iron sledge with a frame, like the horizontal 

 circle. 



Between the eye and the analyzer I brought a flint-glass prism 

 of from 30° to 60° refracting angle with a vertical edge, or a 

 system of prisms for direct vision. The light from the slit of 



Y2 



