324 M. G. Quincke on the prime Angle of Incidence 



the collimator gives then a horizontal spectrum with Fraunho- 

 fer's lines of from 1° to 3° breadth. 



On the table of the horizontal circle, instead of the plane 

 glass, two parallel plane mirrors of the substance to be examined 

 are fastened with their ends overlapping, so that one of them by 

 reflection of the cross-threads can be placed perpendicular to 

 the axis of the auxiliary telescope or the collimator, and its 

 parallelism be tested by turning it through 180°. The mirrors 

 were, by means of the arrangement described in Pogg. Ann. 

 vol. cxlii. p. 198, pi. 5. fig. 3, placed parallel, being fastened 

 with wax on the two sledges, the faces pressed against each other, 

 and the sledges drawn asunder. 



Frequently they were only fastened with wax upon a horizontal 

 glass plate, and one of the mirrors rotated until the twice re- 

 flected image of a distant object (a tree on the horizon) was coin- 

 cident with the object itself. 



If from a heliostat sunlight be let through the slit of the col- 

 limator, and polarized in the azimuth +45°, to the parallel mir- 

 rors, and the light reflected under the prime angle of incidence 

 be taken up by the analyzing Nicol in the azimuth + /3, a dark 

 streak appears in the spectrum, which can be made to fall on a 

 determined Fraunhofer line by changing the angle of incidence 

 H and the azimuth /3. The reading of the incidence-angle gives 

 directly the prime angle of incidence H for the Fraunhofer line 

 in question, and equation (2) the corresponding prime azimuth. 



From the determinations with positive and negative angles of 

 incidence and positive and negative azimuths the mean was 

 taken. 



The defects of the method consist in the difficulty of obtaining 

 Nicol prisms with even surfaces, which, brought in front of the 

 objective of the collimator and the telescope, would give distinct 

 images of the slit, and, further, that the incident and the issuing 

 ray make with each other an angle which may amount to as much 

 as 1°. For this reason I have mostly omitted the observing 

 telescope between the eye and the analyzer. 



On turning the polarizing Nicol from the azimuth +45° to 

 — 45° the image of the collimator-slit was displaced towards the 

 cross-thread of the auxiliary telescope —8', as shown by the 

 image of the thread reflected by the plane glass. This alteration 

 of the angle of incidence — 8' had to be taken into account in 

 the determination of H. 



The divisions of the circle were read with verniers accurately 

 to 1'. The error of a single determination of H or (3 scarcely 

 amounts to 1', if the reflecting surface is even and homogeneous 

 and has at all parts the same optical constants H and B. 



Variations of temperature, however, and pressure in polishing 



