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the two surfaces meet at an angle and form a wedge or prism and the 

 light is directed against one surface obliquely, a part of the light passes 

 through the prism, but in doing so it is not only bent out of its course 

 but it becomes analyzed, and those rays such as the blue, which are the 

 result of shorter and quicker waves, are more bent or refracted than 

 others, such as the red and yeUow, produced by longer and slower vibra- 

 tions. Newton discovered this fact and propounded it exactly 201 years 

 ago in the form of the celebrated theorem that "lights which differ in 

 colour differ also in their degree of refrangibility." His mode of proof 

 was as follows. — Through a small hole in the shutter he allowed a ray of 

 light to pass into a darkened room, forming a spot of white light upon 

 the opposite wall. He then interposed a prism of glass so that the ray of 

 light fell obliquely upon its surface and was delighted to find the spot of 

 white light replaced by a band of most gorgeous colours. 



Newton termed the band of light so obtained the spectrum. I cannot 

 shew the spectrum of sun light, but I will endeavour to show you the 

 spectrum of a white light, the electric light, which rivals that of the sun 

 in intensity. You will observe that the spectrum is continuous, and the 

 colours shade off gradually into one another. Newton saw no more than 

 this when he examined sunUght, because he admitted his light through a 

 round hole. But Wollaston on admitting the light through a narrow sUt 

 found that the spectrum was crossed by a number of very fine dark lines, 

 and Fraunhofer carefully mapped and lettered these lines which have 

 since borne his name. Kirchoff and Bunsen employed this plan for 

 examining coloured flames, and instead of employing a dark room and 

 shutter with a sUt, constructed the instrument termed the spectroscope. 

 I have here the instrument in one of its earliest forms. You will observe 

 that it consists essentially of a triangidar prism of glass, fixed firmly upon 

 a table. Opposite one face of the prism there is a tube, at the end of 

 which is an arrangement for admitting a ray of light through a very nar- 

 row slit. This ray of light strikes the face of the prism obliquely, and 

 in passing through the glass it is both refracted and decomposed, and can 

 be seen through the opposite face of the prism. In order to view the ray 

 more accurately a telescope is placed opposite this face. Two fine cross 



