II. SELECTORS. 219 



with the tube, and the analysing Nicol over which there is a slit or a square 

 aperture. 



At the end of the tube is a deflection prism of crown glass, to which, for 

 some investigations, a direct vision prism is added. The raj, as it issues from 

 the tube, is received by a lens, which throws upon a screen a sharp image of 

 the slit or square aperture. This image moves in a circle as the azimuth 

 changes, and thus shows by quick rotation all the phenomena which, in 

 ordinary polarising instruments, appear successively side by side. 



868. Twelve Plates with Pictures, of gypsum and mica, 

 lor polarised light. Prof. Karsten, Rostock. 



The form of images has been chosen to repi'esent the different colours of 

 thin plates in polarised light. Any kind of polarising apparatus may be 

 employed for these observations. 



869. Norremberg's Polarising Apparatus, small size. 



W. Apel, Gottingen. 



870. Worremberg's Polarising Apparatus, large size ; 

 according to the design of Professor Listing. 



W. Apel, Gottingen. 



The apparatus serves not only for purposes of lecture demonstration, 

 but also for accurate measurements. The advantage of the instrument over 

 the ordinary polarising microscopes lies in the circumstance that in the 

 Norrembery apparatus the polarised light passes to and fro through the same 

 crystal plate, when placed on the horizontal mirror. The movable glass 

 plate of the middle table serves for measuring the angle of the optical axes 

 by means of a graduated semicircle. 



87Oa. Large Apparatus by Norremberg, improved by Wheat- 

 stone. M. Luiz, Paris. 



87Ob. Apparatus used for observing the Polarisation of 

 Light in Water. J. Louis Soret, Geneva. 



It is formed of a telescope tube closed on the objective side by a glass 

 plate. The eyepiece is formed of a " Nicol " prism. 



The observer, placed in a boat, immerses the objective end of the tube 

 and looks through the " Nicol." He then finds the light of blue coloration 

 reflected by the lower strata under the surface of the water, and by turning 

 the Nicol ascertains if it is polarised. 



See " Notes sur la Polarisation de la Lumiere de 1'Eau." Archives des 

 Sciences physiques et naturelles, 1869, Vol. 35, p. 84, and 1870, Vol. 39. 

 p. 352. 



871. Apparatus for the Observation and Measurement 

 of the cyclopolar double refraction of Quartz in the 



direction of the optical axis. Designed by Professor Listing, 

 executed by R. Winkel in Gottingen. 



Royal Mathematical and Physical Institute of the Univer- 

 sity of Gottingen, Prof. Listing. 



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

 for distant 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 



