180 SEC. 7. LIGHT. 



In practice, however, it is found necessary to vary the striking distance 

 with the composition, and the amount of this variation is still under investi- 

 gation. 



826. Spectroscope for determining the smallest displace- 

 ment of spectral lines, and for measuring velocity of motion. 



Professor Carl Wenzel Zengcr, Prague. 



This new instrument gives double images, two spectra produced by an 

 additional prism of quartz or calcspar, giving two dark lines in parallel 

 directions, e.g., the D. line, and of constant distance, if there be no motion 

 towards or from the luminous body. The motion of heavenly bodies pro- 

 ducing, therefore, the displacement of both D lines, and ah accurate micro- 

 meter measuring it, gives the amount of velocity. 



827. Hermann's Taomatoscope, for examination and 

 demonstration of absorption bands in fluids by the spectroscope. 



Professor Dr. L. Hermann, Zurich. 



The fluid is poured into the little chamber, and the thickness of the layer 

 is regulated by sliding the inner tube until the bands appear. 



828. The Collection of Prisms of crown and flint glass used 

 in the construction of refractors and spectroscopes by Steinheil and 

 Merr at Munich, and by Hofmann at Paris, whose refractive 

 indices for 50 lines in the solar spectrum were determined by 

 Prof. Van de Willigen. Foundation Teylcr at Haarlem. 



Steinheil No. I. flint, No. II. flint, No. III. crown glass. 



Merr No. I. and No. II. both of the same heaviest flint, No. III. crown, 

 No. IV. crown, No. V. and No. Va. both of the same ordinary flint glass. 



Hofmann No. I. heavy flint glass. 



See " Archives du Musee Teyler, " Vol. I. p. 31, 64 and 205, and Vol. II. 

 p. 183. 



See the chemical composition of crown Steinheil No. III., and Merr 

 No. IV., and of flint : Steinheil No. II., Merr No. I. and No. II., and 

 No. V. and No. Va., and Hofmann No. I., given by Prof. P. J. van Kerckhoff, 

 " Archives du Musee Teyler," Vol. III. p. 117. 



Steinheil No. II. and No. III., Merr No. I. and No. II., No. IV. and 

 No. V. and No. Va., and Hofmann No. I. are accompanied by parallelepipeds 

 and plates of the same glass and by pieces or powder for chemical analysis. 



829. Powerful Spectroscope, with Browning's automatic 

 action, for adjusting the prisms to the minimum angle of deviation 

 of the ray under examination. John Browning. 



In this instrument the ray can be made to pass four times through the six 

 prisms, and a dispersive power of 24 prisms thus obtained can be used, or 

 that of any lesser number of prisms at pleasure. The instrument is fitted 

 with a new reflecting bright line micrometer ; when measuring with this 

 contrivance no light is visible in the field of view, but the wires of the 

 micrometer are seen faintly illuminated. 



830. Universal Spectroscope, with Browning's automatic 

 action, giving a dispersive power of from 2 to 12 prisms. 



John Browning. 



