CONSTRUCTION OF A SPECTROSCOPE. 501 



which are glued to the bottom in close contact with the prism, 

 which not only is thus secured in its position, but may be at once 

 again inserted into its right position after it has been taken out. 



During use, the box (the lid of course being closed), is placed upon 

 some convenient support, so as to have the flame to be investigated 

 at the same height as the slit. 



In fig. 280, C to J&, / is a wooden piece to be prepared by a 

 turner ; it should have a broad foot 15 cm in diameter, and on the 

 top a hollow box, 8 cm wide inside, 6 cm deep, and the sides should be 

 from 5 mm to 10 mm thick ; the box is closed by a cap d (fig. C and N). 

 The bottom of the box should be about 15 cm above the base of the 

 foot. If the prism is smaller than the one in fig. 277, the box may 

 have correspondingly smaller dimensions. If it is intended to 

 varnish or polish the wood, this must be done after the proper place 

 for the eye-hole has been found by the method previously described. 

 The hole is bored 15 mm wide, and the outside of the box then made 

 flat with the plane, as shown in the figure, so as to make the edges 

 of the hole very thin. The slit is prepared as above described. The 

 glass with the tinfoil is fixed over the aperture cut out of a round 

 piece of cardboard, 5 or 6 cm in diameter, which is attached to one end 

 of a tube of cardboard, of which the other end is glued into a suitable 

 hole in the side of the box. It is very useful to make the cardboard 

 tube of two parts which slide one in the other, each part being 25 cm 

 long ; the distance between slit and prism may then be altered at 

 will. The narrower tube should in that case be 2'5 or 3 cm wide. 

 The proper position of the prism is secured by gluing to the bottom 

 of the box three strips of pasteboard or wood ; the interior of box 

 and cap and the portion round the aperture for the eye are blackened, 

 and the tubes lined inside with dull black paper. 



It is best to load the foot with lead. To this end a circular groove, 

 a section of which is seen from E, is turned into the foot and lead 

 poured into it (400 to 500 grammes). If the wood of which the 

 foot is made is very dense and heavy, it will have sufficient stability, 

 in spite of the projecting tube, without the lead. 



A hydrogen flame is best for producing the coloured vapours, on 

 account of its high temperature ; where coal-gas is accessible, the 

 flame of a Bunsen's burner may be used, but the spectra obtained 

 are in that case not quite so beautiful. 



The spectroscope produces, as has already been 

 mentioned, images of the slit that is, thin lines of 

 various colours. Thus compounds of sodium give a 



