NATURAL PHILOSOPHY. 



135 



The lines of the spectrum show themselves most plainly when the 

 temperature of the flame is highest and its illuminating power least; 

 hence a hydrogen gas burner, which gives a feeble illumination with 

 great heat, is b'est adapted for the purpose of experimenting. 



The apparatus employed by Messrs. Kirchhoff and Bunsen in their 

 observations is thus described in Poggendorff's Annalen. (See figure.) 



A is a box blackened on the inside, having its horizontal section in 

 the form of a trapezium, and resting on three feet ; the two inclined 

 sides of the box, which are placed at an angle of about 58 from each 

 other, carry the two small telescopes B and C. The eye-piece of the 

 first telescope is removed, and in its place is inserted a plate, in which 

 a slit made by two brass knife-edges is so arranged that it coincides 

 with the focus of the object-glass. The gas lamp D stands before the 

 slit in such a position that the mouth of the flame is in a straight line 

 with the axis of the telescope B. Somewhat lower than the point at 

 which the axis of the tube produced meets the mouth, the end of a 

 fine platinum wire bent round to a hook is placed in the flame. The 

 platinum wire is supported in this position by a small holder, E, and 

 on to the hook is melted a globule of the metal or salt of the metal 

 which it is desirable to examine. Between the object-glasses of the 

 telescopes B and C is placed a hollow prism, F, filled with bisulphide 

 of carbon, and having a refracting angle of 60; the prism rests upon 

 a brass plate, movable about a vertical axis. The axis carries on 

 its lower part the mirror G, and above that the arm H, which serves 

 as a handle for turning the prism and mirror. A small telescope 

 placed some way off is directed toward the mirror, and through this 

 telescope an image of a horizontal scale fixed at some distance from 

 the mirror is observed. By turning the prism round, every color of 

 the spectrum may be made to move past the vertical wire of the tel- 

 escope C, and any required position of the spectrum thus brought to 

 coincide with the vertical line. Each particular portion of the spec- 

 trum thus corresponds to a certain point on the scale. If the lumi- 

 nosity of the spectrum is very small, the wire of the telescope C may 



