192 s K<:. 7. LIGHT. 



884. Wheatstone's Polar Clock. To determine the true 

 solar time by the polarisation of light reflected from the sky. 



Wheatstone Collection of Physical Apparatus. King's 

 College, London. 



885. Latest form of Wheatstone's Polar Clock. To 



determine the true solar time by the polarisation of light 

 reflected from the sky. 



Wheatstone Collection of Physical Apparatus, King's 

 College, London. 



886. Polariscope and Set of Crystals. 



Prof. W. F. Barrett, Dublin. 



887. Norremberg's Polarising Apparatus, with Wheat- 

 stone's improvements. H. Lloyd \ Trinity College, Dublin. 



888. Duboscq's Polariscope, for determining the incli- 

 nation of the axes in biaxial crystals. 



H. Lloyd, Trinity College, Dublin. 



889. Wheatstone's Apparatus for illustrating the laws of 

 interference of polarised light. 



H. Lloyd, Trinity College, Dublin. 



III. PHOTOMETERS. 



891. Great Atmospheric Photometer. A De la Rive model, 

 designed by M. Thury, and constructed by the Geneva Associa- 

 tion for Constructing Scientific Instruments, 



De la Rive Collection. The property of Messrs. Soret, 

 Perrot, and Sarasin, Geneva. 



This apparatus is particularly intended to measure the transparency of the 

 atmosphere. It is used for simultaneous observation, with one eye, through 

 two similar reflectors placed at different distances. The difference of bright- 

 ness and of tint between the two reflexions indicates the effect of the inter- 

 posing stratum of air. The computation of this difference is arrived at by 

 equalising the two reflexions by means of diaphragms of .different aperture, 

 and of glass plates variously tinted. The instrument is composed of a 

 telescope with a single eye-piece, and two objective tubes, of which the 

 angular distance varies at will between and 29. A system of four full 

 reflecting prisms unites the two divergent cones in the eye-piece. The 

 apparatus is movable round three different axes, and may be worked in the 

 most varied directions. Graduated circles measure these diverse angular 

 motions. It is a general photometer, and can be specially used as an astro- 

 nomical photometer. De la Rive has effected with it a long series of obser- 

 vations on the transparency of the air. (See Comptes Eendus, vol. 63, 

 p. 1221.) 



892. Photometer, according to Glan's system, for photome- 

 trical determination of the absorption spectra for homogeneous 

 light. Schmidt and Hacnsch, Berlin. 



