188 SEC. 7. LIGHT. 



The ray of sun or electric lamp falls through a Nicol, which is protected 

 with a shade, upon a press, in which the object is fastened by means of spring 

 clamps, and passes thence through a tube which can be rotated with great 

 velocity. This tube is provided at one end with a shade capable of rotating 

 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 ray, 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, 

 for polarised light. Professor Karsten, Rostock. 



The form of images has been chosen to represent the different colours of 

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

 employed for these observations. 



869. Morremberg's Polarising Apparatus. 



W. Apel) Gottingen. 



870. Norremberg's Polarising Apparatus, large size ; 

 according to the design of Professor Listing. 



W. Apel, Gottingen. 



(2.) 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 

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

 crystal plate. The movable glass plate of the middle table serves for 

 measuring the angle of the optical axis by means of a graduated semi- 

 circle. 



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

 stone. M. Lutz, Paris. 



870b. Telescope used for observing the Polarisation of 

 Light in Water. J. Louis Soret, Geneva. 



The telescope is closed on the objective side by a glass pane. The eye- 

 piece is formed of a " Nicol " prism. 



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

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

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

 Nicol ascertains if it is polarised. 



See " Notes sur la Polarisation de la Lumiere de PEau." 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. 



