70 LECTURES TO SCIENCE TEACHERS. 



refrangibility almost without apparatus, by using your 

 darkened chamber with a piece of blue glass in the window. 

 Suppose that in front of the blue glass you place a piece of 

 white earthenware, such as a saucer turned upside down. 

 If you hold a slit at arm's length and view it through a 

 prism, in the first instance aiming at the blue glass and 

 looking up at the sky, you will see the sort of light trans- 

 mitted. The brighter parts of the original spectrum will be 

 almost entirely wanting, but you will see the violet, much 

 of the blue, and the faint extreme red which is freely trans- 

 mitted, and if the glass be not very deeply coloured a little 

 faint greenish yellow which is not yet wholly absorbed. 

 If you now aim at the white plate instead of the sky, you 

 will see just the same spectrum as before, only not quite so 

 strong. Now suppose you lay on the white plate a little 

 bit of ordinary scarlet cloth, hold the slit close to that, and 

 aim at both the cloth and the white plate, so as to get 

 from different parts of the slit a spectrum of the light 

 coming from each. This particular cloth in the blue field 

 would look red On examining the joint spectrum, the part 

 seen by projection of the slit on the plate will appear as 

 just described, while that seen by projection on the scarlet 

 cloth will show a prolongation of the extreme red, and a great 

 deal of bright light where there is none in the incident light, 

 while the violet part will be nearly black. 



This diagram, which was made for another purpose, may 

 serve to exhibit what you would see in that particular case. 

 It really represents what you see by looking through a 

 crystal of nitrate of uranium placed immediately behind 

 the slit with which you aim at the white light sifted 

 through a blue medium. There are certain bands of 

 absorption where there is a maximum of opacity in the 

 incident light ; and when you analyse the beam of light 

 which comes through you see in the transmitted rays there 

 are certain dark bands of absorption ; but over and above 

 that, there is light created with a refrangibility less than 

 exists at all in the incident beam, and in the particular case 

 of the salt of uranium the prismatic composition of this light 

 is very peculiar, its spectrum consisting of bright bands. 



Now as I want to show you how to make experiments 

 yourselves without apparatus, I may mention, that suppos- 

 ing you have one of these solutions in a test tube, you very 



