II. SELECTORS. 209 



heights and intensities of the lines of the two metals vary. For these com- 

 parisons the gold line having a wave length of 5,230 tenth-metres, and the 

 copper line 5,217, are the most convenient. If a series of known alloys vary- 

 ing slightly in composition is examined, a curve may be constructed, the ordi- 

 nates of Avhich represent the ordinary assays, and the abscissae the micrometer 

 readings for the points at which the above two lines are equally bright, and 

 then, theoretically, if an unknown alloy of about the same composition be 

 examined, this curve enables us to determine its exact composition when the 

 micrometer reading is known. 



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

 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 the velocity of motion of 

 the object. 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 an accurate micro- 

 meter measuring it, gives the amount of velocity. 



827. Hermann's Haematoscope, 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 

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

 indices for 50 lines in the solar spectrum were determined by 

 Prof. Van der Willigen. Teylcr Foundation, Haarlem. 



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



Merz 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. 



Ilofmann 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 Steiuheil No. III., and Merz 

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

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

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



Steinheil No. II. and No. III., Merz 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 lo pass four times through the six 

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

 4007.. 



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