196 POPULAR LECTURES AND ADDRESSES. 



you see one diffraction spectrum by reflection from 

 one of Rutherford's gratings, in which there are 

 17,000 lines to the inch on polished speculum- 

 metal. The explanation by "interference" is 

 substantially the same as that which the un- 

 dulatory theory gives for Newton's rings of light 

 reflected from the two surfaces, which you have 

 already seen. Where light-waves from the aper- 

 tures between the successive bars of the grating 

 reach the screen in the same phase, they produce 

 light ; there, again, where they are in opposite 

 phases, they produce darkness. 



The beautiful colours which are produced de- 

 pend on the places of conspiring and opposing 

 vibrations on the screen, being different for light- 

 waves of different wave-lengths ; and it was by the 

 measurement of the dimensions of a diffraction 

 spectrum such as the first set you saw (or of finer 

 spectrums from coarser gratings) that Fraunhofer 

 first determined the wave-lengths of the different 

 colours. 



I have now, closely bearing on the question of 

 the size of atoms, thanks to Dr. Tyndall, a most 



