PERIODICITY 



7i 



of passing on in a straight line, it spreads out like a fan, 

 forming a diverging cone of light, just as if the orifice were 

 itself a luminous point. This is the phenomenon of diffraction 

 which has hitherto heen considered incompatible with the 

 emission theory of light. Diffusion waves may also be made 

 to pass through a narrow orifice, when they will behave 

 exactly like the waves of light. The new waves radiate from 

 the orifice like a fan, instead of giving a cone of waves 

 bounded by lines passing through the circumference of the 

 orifice and the original centre of radiation. Thus on passing 

 through a small orifice 

 diffusion waves exhibit 

 the phenomenon of dif- 

 fraction just as light 

 waves do. 



Interference. — The 

 phenomenon of inter- 

 ference may also be 

 illustrated by waves of 

 diffusion. If on a gelatine 

 plate we produce two 

 series of diffusion waves 

 from two separate centres, 

 we get at certain points 

 an appearance corre- 

 sponding to the inter- 

 ference of two sets of light waves. This appearance is best 

 shown by sowing on the gelatine film a straight row of 

 drops equidistant from one another. It should be remarked 

 that this phenomenon of the production of circles of pre- 

 cipitate separated by transparent spaces, although periodic, 

 is not of necessity vibratory or undulatory. It would thus 

 appear that periodic phenomena maybe propagated through 

 space without vibratory or oscillatory motion. If we submit 

 to a critical examination the various experiments which have 

 established the undulatory theory of light, we find that they 

 do indeed demonstrate the periodic nature of light, but in no 

 wise prove that light is a vibratory movement of the ether. 



Fig. 15. — Diffraction oi diffusion waves on 

 passing through a narrow aperture. 



