62 LECTURES TO SCIENCE TEACHERS. 



having a locus in space, and that locus in space of one of the 

 dark lines is what you may call a dark plane. At a con- 

 siderable distance from the focus it would diverge out, 

 forming a sort of wedge. These dark planes are seen as 

 interruptions to the blue light. "What are the Fraunhofer 

 lines 1 They are parts of the spectrum when the light is 

 missing, and consequently any effect the light is capable of 

 producing will be missing too. Therefore when we get to 

 the invisible region, if the invisible light is missing the 

 visible light which that might be capable of producing will 

 be missing too, and therefore you will see interruptions in 

 this continuous mass of blue light. It constitutes a very 

 striking experiment with sunlight, when you form an 

 approximately pure spectrum by placing some prisms close 

 to a pretty broad slit, and take a tube filled with the solu- 

 tion of quinine, or a prepared solution from horse chestnut 

 bark, and make it pass through the difierent parts of the 

 spectrum in succession, beginning at the red end. At 

 first it looks like water by transmitted light ; the light 

 rays are transmitted through it as they would be through 

 water, on to the blue, but when we get on to the violet 

 then the whole of the tube is lit up with this faint 

 ghostly sort of blue light ; when you have got beyond the 

 visible rays altogether the tube is still lit up with the 

 blue light. This shows that the explanation which occurred 

 to me is really the true one, and that this blue colour is 

 produced, not by the blue rays of the spectrum at all, but 

 by other rays altogether ; that rays of one refrangibility 

 act on the fluid in such a manner as to cause it to give 

 out rays of a different refrangibility altogether, or rather, 

 of a different series of refrangibilities, because if you 

 examine a small portion of this blue light, produced by 

 rays of one definite refrangibility only, as you may do by 

 putting the solution in a pure spectrum and placing a slit 

 in front, so as to let only a narrow strip of the incident 

 rays shine on the fluid, and then analysing the narrow beam 

 from above by a prism applied to the eye, you will find 

 that the light is not homogeneous at all. Nor again is there 

 anything in the phenomenon which recalls to the mind the 

 acoustic phenomenon of harmonics ; the light is perfectly 

 heterogeneous. 



I have on one of these diagrams another mode exhibited 



