II. SELECTORS. 189 



The telescope can, before being put into the holder of the apparatus, be adjusted 

 for far off objects, or for an object of but 2-3 meters distance from the object 

 glass. The Fresnel triple quartz prism is fixed in upright position in the 

 support below the telescope, and protected by a cardboard shade against side 

 lights. By means of the achromatic lens, situated below the prism, a virtual 

 image is produced of an appropriate object (line cut by a diamond upon glass, 

 &c.)> placed upon the black table, which image, when seen sharp and double 

 in the telescope, will be just as distant from the object glass of the telescope 

 as the latter has been adjusted for. The magnitude of the duplication is read 

 off from the micrometer in the ocular tube, and from the number obtained the 

 diversion of the two rays after their passage through the triple prism is 

 calculated. The ocular can now be provided with tourmaline and % mica 

 plate, which may be used singly or combined. The tourmaline alone, shows, 

 on turning, in all azimuths, the double image without alteration of intensity 

 in the component parts ; the two rays undergo, therefore, neither linear (plain) 

 nor elliptical polarisation. The tourmaline, with mica plate below it, shows, 

 as well known, that both rays are circularly polarised, the one right, the 

 other left; the tourmaline must in this case be so adjusted that its line of 

 principal action be azimuth or 90, and the main section of the interposed 

 mica plate is turned to form with that line + 45. 



The aim of the measurements is to determine the refraction indices of the 

 two rays of opposite circular polarisation, propagated with unequal velocity 

 along the optical axis of the quartz. 



872. Stauroscope, according to the design of F. von 

 Kobell, executed by Bochur and Wiedemann. 



Prof. Dr. Franz von Kobell, Munich. 



873. Analysing Prism of Iceland Spar, made by the 

 inventor, the late William Nicol, in his 80th year. 



Edinburgh Museum of Science and Art. 



The Nicol prism is so constructed that only one polarised ray can pass 

 through it. 



873a. A Nicol's Prism for Polarising Light, by C. D. 



Ahrens. W. Spottiswoode, F.R.S. 



This, which is one of the largest ever constructed, has a clear field of 3^ 

 inches in diameter. With a view to saving bulk and weight, the acute angles 

 have been cut off, and the whole reduced to an octagonal form. The 

 advantages of this will readily be seen by comparing this instrument with that 

 by Tisley and Spiller, the field of which is greater by only a quarter of an 

 inch. 



873b. A MicoPs Prism for Polarising Light, by Tisley 

 and Spiller. W. Spottiswoodc, F.R.S. 



This, which is the largest and purest ever constructed, has a clear field of 

 fully 3^ inches in diameter. 



874. Soleil-Ventzke Polarising Apparatus, with several 

 improvements. Franz Schmidt and ffaensch, Berlin. 



Soleil having introduced compensation by the use of rock-crystal, Ventzke 

 subsequently improved the colour-giving power, and Scheibler made further 

 improvements, principally in the manner of inserting the observation tubes. 

 Messrs. Schmidt and Haensch, besides a few minor changes, succeeded in 



