6o4 THE POPULAR SCIENCE MONTHLY. 



This and other reasons compel me to hold that the answer to the 

 question put is, that what has been taken for granted is, in all proba- 

 bility, not true. But before I proceed to give the reasons for the faith 

 that is in me I must, at the risk of being both technical and tedious 

 when I should wish to be neither, lead up to the understanding of them. 



The spectroscope, however simple or complex it may be, is an in- 

 strument which allows us to observe the image of the slit through which 

 the light enters it, in the most perfect manner. If the light contains 

 rays of every wave-length, then the images formed by each will be so 

 close together that the spectrum will be continuous, that is, without 

 break. If the light contains only certain wave-lengths, then we shall 

 get certain, and not all, of the possible images of the slit, and the spec- 

 trum will be discontinuous. 



Again, if we have an extremely complex light-source, let us say a 

 solid and a mixture of gases giving us light, and we allow the light to 

 enter, so to speak, indiscriminately into the spectroscope, then in each 

 part of the spectrum we shall get a summation — a complex record — of 

 the light of the same wave-length proceeding from all the different 

 light-waves. But if by means of a lens we form an image of the light- 

 source, so that each particular part shall be impressed in its proper 

 place on the slit-plate, then in the spectrum the different kinds of light 

 will be sorted out. 



There is a simple experiment which shows clearly the different re- 

 sults obtained. If we observe the light of a candle with the spectro- 

 scope in the ordinary manner, that is, by placing the candle in front of 

 the slit at some little distance from it, we see a band of color — a contin- 

 uous spectrum — and in one particular part of the band we see a yellow 

 line, and occasionally in the green and in the blue parts of the band 

 other lines are observable. Now, if we throw an image of the candle 

 on to the slit — the slit being horizontal and the image of the candle ver- 

 tical — we then get three perfectly distinct spectra. We find that the 

 interior of the candle, that is the blue part (best observed at the bottom 

 of the candle), gives us one spectrum, the white part gives us another, 

 while on the outside of the candle, so faint as to be almost invisible to 

 the eye, there is a region which gives us a perfectly distinct spectrum 

 with a line in the yellow. In this way there is no difficulty whatever in 

 determining the coexistence of three light-sources, each with its proper 

 spectrum, in the light of a common candle. 



"We see in a moment that much the same condition of affairs will be 

 brought about if, instead of using a candle, we use an electric arc, in 

 which the pure vapor of the substance which is being rendered incandes- 

 cent fills the whole interval between the poles, the number of particles 

 and degree of incandescence being smaller at the sides of the arc. We 

 can throw an image of such an horizontal arc on a vertical slit; the slit 

 will give then the spectrum of a section of the arc at right angles to its 

 length. The vapor which exists farthest from the core of the arc has a 



