Spectroscope Apparatus. 243 



ment, of two and a half inches aperture, and three feet focal 

 length. These show many additional lines. With this instru- 

 ment a very careful survey and mapping of the spectrum is 

 being carried on, and the results will, no doubt, be the most 

 precise and complete information concerning the number and 

 position of the dark lines that has yet been obtained. The size 

 of the Gassiot spectroscope enables it to give a bright image 

 with a small allowance of light ; but certain inquiries in which 

 Mr. Huggins is engaged need a further extension of power in 

 this direction, and Mr. Browning is constructing for him a 

 monster instrument, with about half the number of prisms on 

 Mr. Gassiotfs instrument; but these prisms fully double the 

 size of Mr. Gassiot' s, and furnished with telescopes of four 

 feet focal length, and proportionably large aperture. With 

 this splendid and costly apparatus — which has quite novel and 

 special means for increasing the dispersion when deemed 

 necessary — it will be possible to obtain great separation with a 

 minimum loss of light, so that the spectra of very feebly 

 luminous bodies may be made out. 



In these two great spectroscopes the extra separation is 

 produced by the multiplication of glass prisms, and when 

 these are of fine quality and exquisitely wrought, the definition 

 is the sharpest that can be obtained. 



Different substances vary in their refractive and dispersive 

 powers. Sir J. Herschel observes, " m general, high refractive 

 is accompanied by high dispersive power ; but exceptions are 

 endless, especially among the precious stones, of which the 

 diamond affords a striking instance." 



If the two powers had always gone together, we could have 

 had no achromatic lenses ; but happily by selecting different 

 kinds of glass, and setting them to opposite work, we can correct 

 all (or nearly all) the error arising from dispersion, and only 

 neutralize a portion of the refraction. When greater dispersive 

 power is required in a prism than ordinary glass exerts, another 

 kind of glass may be chosen, or recourse may be had to a 

 liquid like bisulphide of carbon, which gives a very wide 

 spectrum. A liquid has the disadvantage that it cannot be 

 maintained in the prismatic form except by putting it in a 

 vessel of the required shape. Hollow prisms are accordingly 

 made of thin glass, and filled with the fluid required. Mr. 

 Browning has made a spectroscope of this kind for Mr. Gassiot. 

 It consists of eleven prisms, with telescopes two and a half 

 inches aperture and three feet focal length, and it is able to 

 separate the principal soda lines to the extent of 3' 6", the 

 nine glass prisms only separating them 1'. The definition is 

 excellent while the fluid is of uniform temperature and density ; 

 but becomes very bad as soon as the heat which accompanies 



VOL. VI. — NO. IV. E 



