II. SELECTORS. 181 



831. Spectrometer, straight sighted, with apparatus for 

 registering observations. 



Geneva Association for Constructing Scientific Instru- 

 ments. 



This straight-sighted spectrometer is distinguished from others of the same 

 description, in that the distance of the rays of the spectrum is measured, not 

 by the superposition of the spectrum sighted upon a lighted micrometric 

 scale, but by measuring the angle formed by the eye -piece for marking the 

 reticle of the telescope from one line upon the other, and then comparing with 

 the angle formed by the telescope pointed on two lines of known distance. 

 A tangent screw effects the angular displacement of the telescope ; this 

 screw bears a calculator which serves to read angular displacements of less 

 than 20 seconds. A recorder, formed of a movable pencil which acts upon 

 a; counter, serves to make series of observations, in the dark. The telescope 

 of the line of collimation which bears the slit, possesses also an angular 

 motion which serves to bring any portion whatever of the spectrum in the 

 centre of the compass of the eye -piece. 



832. General Apparatus for Spectroscopy, Polari- 

 sation, Reflexion, Refraction, and for various experiments 

 in Fluorescence. 



Geneva Association for Constructing Scientific Instru- 

 ments. 



This apparatus has been constructed with the object of carrying out, with 

 one and the same instrument, all, or nearly all, experiments in spectroscopy, 

 rotatory polarisation, reflection, and refraction. The divided circle is mov- 

 able around an axis, and serves to bring the rays of light at any angle upon 

 the eye-piece of the line of collimation. For experiments in spectroscopy, a 

 table of one, three, or six prisms may be set up. The prisms are raised above 

 the divided circle sufficiently to allow of their being heated from below if 

 required. For determination of the line of collimation suitable arrangements 

 have been provided, and for experiments in polarisation, eye-pieces fitted with 

 divided^ circles and nicols ; a Babinet compensator may also be adapted to them. 



The first apparatus of this kind was constructed for the use of Professor 

 Wiedemann, of the University of Leipzig. 



833. Large Spectrometer, according to Meyerstein's system, 

 for determining the relations of refraction and dispersion of diffe- 

 rent media, with contrivance for polarisation. 



Schmidt and Haensch, Berlin. 



834. Smaller Spectrometer, of the exhibitors' own con- 

 struction. Schmidt and ffaensch, Berlin. 



835. Spectrometer, according to Abbe's system, with divi- 

 ded circle of 20 cm. diameter, repetition circle, and micrometer 

 apparatus for determining the dispersion. Additional to the same, 

 a hollow prism with metal body. Carl Zeiss, Jena. 



The spectrometer has only one telescope, which serves at the same time 

 as collimator and as instrument of observation. The adjustment brings 

 about automatically the minimum of deviation for every ray. The measure- 

 ment of the refracting angle and that of the deviation takes place without 



