NATURAL PHILOSOPHY. 133 



of producing geometrical figures, causes flashing rays of light to 

 appear and be projected on the screen, with the most extraordi- 

 nary results. By employing variously colored discs of glass or 

 other material with this instrument, the most gorgeous effects of 

 color may be obtained. 



VELOCITY OF LIGHT AND SOUND. 



It has been calculated that the deepest note which the human 

 ear perceives as a continuous sound is produced by 16 vibrations 

 in a second ; the acutest by 48,000. The extremes of color are 

 red and violet, the former given by 458 billions of vibrations per 

 second, and the latter by 727 billions. The relative velocities of 

 light and sound, and the relative refinement of the media through 

 which their effects are conveyed, are illustrated by this compar- 

 ison. 



WAVE LENGTHS OF SPECTRA. 



M. Heinrichs, in the " American Journal of Science," gives the 

 results of his studies on the relations between the wave lengths of 

 absorption bands in the spectra of various elements. Among his 

 conclusions are the following : the wave lengths corresponding to 

 the bands in each elementary spectrum differ, by a fixed number 

 or some simple multiple of the same ; or, in other words, if we 

 suppose lines to have been made at regular intervals, and then 

 some of them obliterated, the actual condition now existing would 

 be reproduced. The dark lines are produced by a certain inter- 

 ference. They are the result of, at most, three systems of inter- 

 ference. The lines generally are closer, the greater the atomic 

 weights of the elements. The distance of the lines is related to 

 the atomic dimensions. 



SPECTRUM ANALYSIS. 



Father Secchi of Rome uses a cylindrical lens, of a focal length 

 of three inches, placed in front of and near the eye-piece ; beyond 

 the lens is placed an Amici prism, in which the deviation is noth- 

 ing; this is a powerful, cheap, and easily applicable arrangement. 

 He so places this spectrometer that the lines in the spectrum are 

 parallel to the celestial equator, or to the direction of the star's 

 apparent motion ; a known or comparison star is then brought on 

 to one of the threads of the finder ; returning then to the large 

 telescope, he brings one of the points of the micrometer behind 

 one of the principal lines of the star's spectrum ; the star to be 

 compared with the first is then brought under the same thread of 

 the finder. If the micrometer point coincides with a line of the 

 spectrum, this line and the line of the first star's spectrum are 

 evidently identical. One of his most remarkable results, if cor- 

 rect, is theobservation that Y Cassiopeise and /? Lyrse show bright 

 lines in the first there are several bright lines, but one dominant 

 line in the blue-green, taking the place of a dark line the line 

 F of hydrogen. This observation seems to indicate that some 



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