xvi.j ARRANGEMENTS ADOPTED. 215 



effectively certain precautions were necessary. A description 

 of the method used will show how these were taken. 



In addition to the lens placed between the lamp and the slit 

 to throw an image of the arc on the latter, another lens was 

 now introduced between the heliostat and the lamp heliostat, 

 lenses, lamp, and collimator being of course in the same straight 

 line. The action of the. newly interpolated lens was to throw 

 an image of the sun between the poles of the lamp, so that 

 when the spectrum of the arc was properly focussed by the 

 camera lens on to the photographic plate, the solar spectrum, 

 when subsequently thrown in, was also in focus. 



So far, then, we had the long and short lines of each 

 substance compared with the solar spectrum on the same plate. 



The accompanying diagrams (Figs. 82, 83, and 84) show the 

 arrangements adopted in the cases mentioned. 



In that branch of the inquiry which dealt with the causes of 

 the coincidences of the lines in various spectra, it was nob 

 essential to refer the lines to the solar spectrum each time ; thus 

 if we studied the cases of, say, aluminium and calcium impuri- 

 ties, it was better to photograph the suspected spectra side by 

 side and confront them. To do this all that was necessary was 

 to extend the application of the principle already referred to. 

 In fact the only practical limit to the number of spectra we can 

 get on to one plate is the time the plate takes to dry, and instead 

 of uncovering half of the slit at one time we may uncover any 

 smaller portion. 



This was effected by means of a brass shutter with a square 

 opening cut through it, which slides in grooves in front of and 

 up and down the slit. On the outside of the shutter holes wtre 

 bored, the distance between each hole being the same as the 

 height of the opening in the shutter. A short pin fixed to a 

 spring falls into each hole in succession as the shutter is moved 

 up or down, and so ensures that there shall be neither super- 

 position of spectra nor gaps between them. 



