242 Spectroscope Apparatus. 



prism which, prevents direct light getting in, "but reflects rays 

 that reach it at right angles to the axis of the instrument. 

 The diagram showing the spectroscope will explain this. The 

 part of the slit not covered by the little prism is supposed to 

 receive light from a source opposite to it, while the light of the 

 candle (shown in the figure) instead of passing straight 

 through the prism, and continuing its course without entering 

 the slit, is reflected from the inner surface of the prism, and 

 thrown through the slit exactly in the right place. Mr. 

 Browning has done wisely in adding this apparatus to his 

 economical spectroscope, and also in providing a convenient 

 holder for a test tube to contain liquids that exhibit Professor 

 Stokes* absorption bands, of which we may say more another 

 time. 



For special purposes it is necessary to view and measure the 

 position of the more delicate lines which the spectrum contains, 

 and to separate some which look single with ordinary means. 

 It would at first seem that the way to do this would be to 

 employ more magnification — to treat, in fact, the close 

 lines as we do those on diatoms, and separate them by 

 magnifying power. Practically this plan only admits of 

 very restricted application ; and in the splendid Gassiot 

 spectroscope, made by Mr. Browning, no deep eye-pieces are 

 employed. Two plans are adopted to get over the difficulty 

 — the spectrum is made wider by greater dispersion — our fan 

 of light is spread out more ; and telescopes of large aperture 

 and greater focal length furnish the spectroscopist with a 

 greater resolving or separating power, the increase of magni- 

 fication being produced by the object glasses instead of the 

 eye-pieces. 



The reader will appreciate the difference between magnify- 

 ing a spectrum of given dispersion and viewing a spectrum 

 of greater dispersion, by remembering our illustration of the 

 fan. Let a fan be opened, so that a portion of the pattern 

 painted on its spokes is concealed by overlapping. It is obvious 

 that masrnifvinsr the fan in this state can ffive us no information 

 concerning the part which is covered up. If we open the fan 

 still more, the concealed parts will come into view. This cor- 

 responds with the additional dispersion that we must give to a 

 spectrum that has been imperfectly spread out, in order to see 

 the entire pattern it can show. 



To spread the light out wider, a multiplicity of prisms 

 maybe employed. The Gassiot spectroscope — the finest instru- 

 ment yet finished of its kind — has nine prisms, about two and 

 ahalf inches long, and two high, and was originally supplied with 

 telescopes of two inches aperture, and two feet focal length. 

 Two fresh telescopes have recently been made for this instru- 



