The Microscope. 51 



THE CONTINUED AND INTERRUPTED SPECTRUM. 



When we examine the flame of an oil lamp, or that of burning 

 magnesium, or the oxygen lime light or the flame of the electric arc, 

 we receive a continuous spectrum; the tints run imperceptibly into 

 each other, as they do in the rain bow. But when sunlight is thus 

 examined, or the light of the moon or the stars, we observe a spectrum 

 interrupted or traversed by many fine lines, some more prominent and 

 distinct than others, but they always appear in the same part of the 

 spectrum. This we call an interrupted spectrum. There are thou- 

 sands of these lines which have been mapped with great care, by 

 Thalen, AngetroL'm and Kirchhof, and their wave-length determined. 



The stars, such as the brilliant Sirius, also give many thousands 

 of lines, but they afe differently ai-ranged, and their prominent lines 

 differ in position and number from those of the sun. The sunlines 

 are known, as we already adverted to, as Frauenhofer's lines, from A 

 to H. They are delineated on the subjoined diagram, which we 

 advise our readers closely to examine and to study. These lines, 

 whether as presented by the sun or stars, represent terrestial sub- 

 stances, especially minerals, metals and gases. They exist in these 

 celestial orbs in an incandescent vaporous condition. The promi- 

 nent E line indicates the presence of iron. C, F and Gr are the 

 lines of hydrogen; calcium has its line at H. We already adverted 

 to the D line indicating the presence of sodium. 



A section of the spectrum and its lines were photographed by 

 our own gifted countryman, Rutherford, and it is astonishing how 

 closely his photograph coincides and agrees with the section of the 

 spectrum, drawn by hand by the immortal Kirchhof. Now, all 

 minerals or metals in a gaseous condition, as well as all other 

 natural gases known to us give a bright line spectrum. Numerous 

 experiments have established this fact. How then can we explain 

 the circumstances, that the sunlines are all black. In answer to 

 this query we have pointed out to you another fact, to- wit, that by 

 the simple device of Kirchhof, the bright lines produced by 

 burning terrestrial metals can be changed into dark ones and vice 

 versa, the dark sunlines can be changed into bright lines, making 

 them identical. From this Kirchhof formulated the following law, 

 in 1860. 



" That the relation between the power of emission, and the 

 power of absorption of the same kind of rays is the same for all sub- 



