390 Celestial Chemistry. 



been arrived at, it is necessary to offer some description of the 

 apparatus employed. In the article on spectroscopes previously 

 referred to, the construction of these instruments has been 

 described, but the modifications necessary for their application 

 to sidereal observations require notice. It is obviously neces- 

 sary to concentrate the light of such faint bodies as the stars 

 before admitting it to the prisms for dispersion, and therefore 

 a telescope of large size became necessary to the observer. 

 Mr. Huggins possesses, in his convenient and well-appointed 

 observatory at Upper Tulse Hill, an instrument having an 

 object-glass of eight inches aperture, and ten feet focal 

 length, made by Alvan Clark, of Cambridge, United States, 

 formerly used with much success by the Rev. W. R. Dawes. 

 This he has had mounted equatorially by Cooke and Sons of 

 York, and this superb instrument, accurately driven by clock- 

 work (an essential condition), forms the condenser of the light 

 of the faint bodies to be submitted to spectral analysis. 

 Another peculiarity of the apparatus requires notice : as a star 

 forms a point of light only in the focus of the object-glass, 

 this would be drawn out by the prisms into a fine line of 

 •coloured light without sufficient breadth for observation of the 

 fine lines crossing it. Some mode of expanding it (and in one 

 •direction only, so as to prevent unnecessary loss of light) to 

 give the required breadth to the spectrum is needed, and a 

 cylindrical lens, as originally employed by Frauenhofer, was 

 found to be the best thing for this purpose. In the accom- 

 panying engraving of the apparatus used by Mr. Huggins and 

 Dr. Miller, the cylindrical lens is marked a. It is plano-convex, 

 about an inch square, and of fourteen inches focal length. It 

 is mounted in a tube b, sliding within the tube c, by which 

 the apparatus is attached to the eye end of the telescope. The 

 slit for the admission of the light is marked d, and over one-half 

 •of it is placed the right-angled prism e, to receive the light 

 for the metallic spectra of comparison. The light of the metallic 

 .sparks is sent by the mirror f through a hole in the tube c to 

 the prism. The achromatic collimating lens is marked g, and 

 the two dispersing prisms of very dense glass h. The spectrum 

 is viewed through a small achromatic telescope marked I, 

 having a positive eye-piece magnifying nearly six: times. This 

 telescope is carried by a micrometer screw q, capable of 

 dividing the space from A to H in the solar spectrum into 1800 

 parts, and the telescope being furnished with a cross of wires, 

 it is obvious the distances of the lines can be measured with 

 extreme accuracy. The arm r carries the wires and forceps 

 connected with the induction coil for deflagrating the metals 

 whose bright lines are examined for coincidence with the dark 

 ones of the stellar spectra. The prisms of this apparatus wore 



