14 SECTION PHYSICS. 



the extent of the slit exposed to the light, and this shutter admits 

 light only through a very small portion of the whole slit. By 

 successively exposing adjoining portions of the slit to the light 

 emitted from the incandescent vapours of metals whose spectra it 

 is necessary to confront with one another fcr the purpose of purification, 

 the final effect on the sensidizcd plate is that two or more spectra suc- 

 cessively record themselves thereon. All superposition of spectra and 

 gaps between them are guarded against by the following means. The 

 brass shutter with the square opening, slides in grooves in front of, and 

 up and down the slit. On one side of the shutter in one of the grooves, 

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

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

 falls into each hole in succession. The scale of measurement adopted 

 is a wave-length scale. In the first instance the solar spectrum as 

 obtained by the means here employed, being a refraction spectrum, is 

 reduced to a wave-length spectrum by means of curves of graphical 

 interpolation ; then the spectra of the metals being photographed side 

 by side with that of the solar spectrum, the measurements of the metallic 

 lines may be deduced from their position in relation to the dark lines 

 of the solar spectrum. Mr. Lockyer then mentioned that observations 

 made during his investigations led him to the conclusion that different 

 molecular aggregations produce five different kinds of spectra, viz. : 



1. Line spectrum. 



2. Channelled-space spectrum. 



3. Continuous absorption at the blue end. 



4. Continuous absorption at the red end. 



5. Continuous absorption. 



In fact modifications in the molecular construction of bodies would 

 produce a corresponding modification in the spectra themselves. Mr. 

 Lockyer also referred to some interesting experiments which he has made 

 in connection with the spectrum of calcium. When, for instance, the 

 chloride of calcium is subjected to a low temperature, we obtain 

 a line in the blue part of the spectrum, together with a nearly 

 complete spectrum of the chloride. This blue line is the blue line of 

 calcium. As the dissociation of the chloride progresses, the blue ray 

 becomes more brilliant and the chloride spectrum gradually disappears. 

 If, instead of the comparatively low temperature hitherto employed, we 



