30 The Ottawa Naturalist. [May 



As many of our deductions will be based on the facts ascer- 

 tained by the spectroscope, it may be as well to briefly explain 

 its principles. The spectroscope has, in its simplest form and 

 as its essential elements, a narrow slit on which the light from 

 the source to be analysed is thrown, a lens behind the slit, 

 called the collimator lens, which renders the light parallel and 

 a prism, a triangular piece of glass, which decomposes or analyses 

 the light into its constituent colors. The spectrum, as the rain- 

 bow colored band which is formed is called, can then be examined 

 with a telescope or photographed by a camera. I have a dia- 

 gram which shows the arrangement of these parts of the instru- 

 ment and I can form a spectrum on the screen. 



The spectrum shown is that of the white hot carbon rods 

 of the electric arc, which give us what is called a continuous 

 spectrum, one in which the colors shade gradually from one to 

 the other. Whenever you see a continuous spectrum, you know 

 that the light source is an incandescent solid or liquid body. 

 If we were to separate the carbon rods and burn a metal or any 

 substance between them, we would get a spectrum of the vapor 

 of that substance which would consist, not of a continuous band 

 of color, but of a number of separated bright lines, distributed 

 over the spectrum, and varying in number from about a dozen 

 in the case of lithium to many thousands in the case of iron. 

 Such a spectrum is called an emission or bright line spectrum 

 and indicates, first of all, that it comes from incandescent gas 

 or vapor, and, secondly, tells us unmistakably the element which 

 produces it. For each element has not only a distinctive and 

 invariable number of bright lines in its spectrum, but the 

 positions and arrangements of these lines are always the same 

 for the same element, and differ for different elements. When 

 these positions are mapped for all the elements, it is evident 

 that by examining the spectrum of any substance, no matter 

 how complex, we can determine the elements of which it is com- 

 posed. There is a third kind of spectrum called an absorption 

 or dark line spectrum, in which the bright lines of the emission 

 spectrum become dark lines in exactly the same positions, and 

 it is evident that the elements producing it can be identified in 

 exactly the same way. The absorption spectrum is produced 

 when an incandescent source shines through gases or vapors at 

 a lower temperature, and is the kind of spectrum given by the 

 majority of the stars, showing that their glowing centres are 

 surrounded by atmospheres of cooler gases. 



Spectrum analysis tells us then, not only what elements 

 any body emitting light is composed of, but also gives us 

 information as to its physical condition, whether solid or gaseous, 

 and whether surrounded by cooler or hotter gases. This is 



