506 Professors Kirchhoff and Bunsen on Chemical 



reflexion from the front surface of the prism. This scale is a 

 photographic copy of a millimetre-scale, which has been pro- 

 duced in the camera, of about T ! j the original dimensions*. The 

 scale is covered with tinfoil so that only the narrow strip upon 

 which the divisions and the numbers are engraved can be seen. 



The upper .half only of the slit is left free, as is seen by refer- 

 ence to fig. 11, Plate VI.; the lower half is covered by a small 

 equilateral glass prism, which sends by total reflexion the light 

 of the lamp D, fig. 12, through the slit, whilst the rays from the 

 lamp E pass freely through the upper and uncovered half. A 

 small screen placed above the prism, prevents any of the light 

 from D passing through the upper portion of the slit. By 

 help of this arrangement the observer sees the spectra of the two 

 sources of light immediately one under the other, and can easily 

 determine whether the lines are coincident or notf. 



We now proceed to describe the arrangement and mode of 

 using the instrument. 



The telescope B is first drawn out so far that a distant 

 object is plainly seen, and screwed into the ring, in which it is 

 held, care being taken to loosen the screws a. and /3 beforehand. 

 The tube A is then brought into its place, and the axis of B 

 brought into one straight line with that of A. The slit is then 

 drawn out until it is distinctly seen on looking through the 

 telescope, and this latter is then fixed by moving the screws a 

 and fi, so that the middle of the slit is seen in about the middle 

 of the field of view. After removing the small spring <y, the 

 prism is next placed on the brass plate, and fastened in the 

 position which is marked for it, and secured by screwing down 

 the spring 7. If the axis of the tube A be now directed 

 towards a bright surface, such as the flame of a candle, the 

 spectrum of the flame is seen in the lower half of the field of the 

 telescope on moving the latter through a certain angle round the 

 axis of the foot F. When the telescope has been placed in 

 position, the tube C is fastened on to the arm belonging to it, 

 and this is turned through an angle round the axis of the foot 

 such that, when a light is allowed to fall on the divided scale, 

 the image of the scale is seen through the telescope B, reflected 

 from the nearer face of the prism. This image is brought 



* This millimetre-scale was drawn on a strip of glass covered with a thin 

 coating of lampblack and wax dissolved in glycerine. The divisions and 

 the numbers, which by transmitted light showed bright on a dark ground, 

 were represented in the photograph dark on a light ground. It would be 

 still better to employ, for the spectrum-apparatus, a scale in which the 

 marks were light on a dark ground. Such scales are beautifully made by 

 Salleron and Ferrier of Paris. 



t This apparatus was made in the celebrated optical and astronomical 

 atelier of C. A. Steinheil in Munich. 



