PERIODICITY 73 



discovery of the cathode rays and the radiations of radium, 

 together with the introduction of the electron theory of 

 electricity, all seem to have brought again into greater 

 prominence Newton's original conception of the emissionarv 

 nature of light. 



Some of the phenomena of radiation can be explained only 

 by the emission theory, and others by the undulatory theory 

 of light. All these difficulties would be solved if we admitted 

 the hypothesis that radiating bodies project electrons, which 

 produce in the ether periodic waves similar to those formed in 

 our gelatine films by the molecules of diffusion. 



These diffusion films are of the greatest possible service in 

 the practical teaching of optics. They place before the eye 

 of the student a working model as it were of the undulations 

 of light. When projected on the screen, they give excellent 

 pictures of the phenomena of refraction, diffraction, and 

 interference, and the simultaneous propagation of undulation 

 of different wave-lengths, and they show in a visible manner 

 the changes of wave-length in media of different densities. 



Diffusion waves differ greatly in length, varying from 

 several millimetres to 2 ft. Many are even shorter than this, 

 too short to be separately distinguished even under the highest 

 power of the microscope, when they give the effect of moire or 

 mother-of-pearl. 



It is easy to construct a spectroscopic grating in this way 

 with fine lines whose distance apart is of the order of a micron, 

 separated by clear spaces. Every physical laboratory may 

 thus produce its own spectroscopic gratings, rectilinear, circular, 

 or of any desired form. 



The most beautiful colour effects may be produced with 

 these diffusion gratings, as we have shown at the Congress of 

 Rheims in 1907. We have a considerable collection of these 

 diffusion gratings, some with very fine lines, giving a very 

 extended spectrum, and others with coarser striatums which 

 give a large number of small spectra. 



This study of periodic precipitates is of the highest interest 

 when we come to investigate the production of colour in 

 natural objects, such as the wings of insects or the plumage of 



