II. SELECTORS. 183 



mining the refraction and the refracting angle ; the small tubes are for the 

 polarisation". Determination of refraction and of refracting angles are 

 effected as with the larger instrument, except that the telescope is put into 

 the place of the micrometric tube. Solid bodies, when submitted to polarisa- 

 tion, are fixed with some wax against the plate which is put up in the black 

 dish. When liquids are to be investigated, the small circle with its clamps is 

 entirely removed, the screw, which maintains the principal circle in horizontal 

 position, taken out, and the instrument turned down until the main circle 

 stands quite vertically, when it is fixed again by the screw. For all polarisa- 

 tion experiments Babinet compensator is fixed by means of the two screws 

 upon the bearer of the telescope. 



839. A hollow Prism, according to Dr. Meyerstein, which 

 is used for optical analysis with the spectrometer. 



Aug. Becker (Dr. Meyerstein' s Astronomical and Physical 

 Workshops), Gottingen. 



840. Rigid Spectroscope by Browning, constructed for 

 Mr. Gassiot on the design of Dr. Balfour Stewart, with the view 

 of determining whether the position of the D lines of the spectrum 

 is constant, whilst the co-efficient of terrestrial gravity is made to 

 vary. Kew Committee of the Royal Society, Kew Observatory. 



It is described in the Proceedings of the Royal Society, Vol. XIV., p. 320. 



Observations were made by it from 1866-1869, on board H.M.S. "Nassau," 

 during a voyage to and from the South Pacific, and subsequently at the Kew 

 Observatory until 1872, the results of which are not yet published. 



It consists of a train of three prisms, the last of which is silvered on one 

 side, so as to return the light which falls upon it. Close to the slit another 

 prism is placed, which reflects the rays on their way back into a micrometer, 

 by which the position of the D lines is measured." 



841. Vierordt Spectroscope, adapted for the measure- 

 ment of absorption spectra and for quantitative chemical analysis. 



Franz Schmidt and Haensch, Berlin. 



This apparatus is described in Vierordt's work on the " Application of the 

 Spectroscope to the Photometry of Absorption Spectra, and in Quantitative 

 " Chemical Analysis." Tubingen, 1873. 



842. Browning Spectroscope, applicable to the exami- 

 nation of absorption spectra, according to Vogel's method. 



Franz Schmidt and Haensch, Berlin. 



843. Mitscherlich's Stand for use in the spectroscopic 

 examination of coloured flames. 



Professor A. Mitscherlich, Milnden. 



844. Pocket Spectroscope, showing sodium line in a 

 simple burning candle, of great use in chemical and meteorological 

 observations. A. Hilger, London. 



845. Hilger's Pocket Spectroscope, showing all the prin- 

 cipal Fraunhofer lines, dividing D easily with nickel line between; 

 when on sun, a sliding slit with division adjustable to position, 

 besides a limb in front of eye-piece. A. Hilger, London. 



