474 



Dr. C. Y. Drysdale on the 



according to size and strength, the reason for dispersion is at 

 once grasped. It is at the same time explained that the more 

 rapid violet light vibrations or shorter waves would naturally 

 be more encumbered by dense or gross matter than the larger 

 red waves. At the same time the general effects of refraction 

 by parallel plates, prisms and lenses are illustrated by the 

 crossing of rivers with parallel, oblique or curved banks. 

 This paves the way for a complete study of refraction and of 

 the properties of prisms and lenses. As it would take up too 

 much space to give a detailed course here, it is proposed 

 simply to give one or two examples as showing either 

 original methods or ones which are less generally known and 

 which the writer considers should be adopted. 



It should be here mentioned that it has been found con- 

 venient when dealing with thin prisms and lenses, to consider 

 the alteration of the wave- front by the prism or wave as a 

 whole, and only to introduce the formulae for single surfaces 

 when thick lenses and prisms or refracting systems in general 

 are considered. 



Retardation by a Parallel Plate. — Since the refractive index 



velocity in air ., ,„ ,, ,, , . . ,, , 



a = — = — - — t— --. - . it follows that m passing through 

 velocity in medium 



a thickness t of glass, the wave would have travelled forward 



a distance fit in air, and the retardation caused by the plate 



is fjut—t=(fjb — l)t. 



Be fraction by a Thin Prism. — In the case of a thin prism 



(fig. 1) having a thickness t at its base and of zero at its 



Fig. l. 



Refraction by Thin Prism. 



apex, the retardation of the wave at the base being (/*— 1)£, 

 we have evidently h — (fi— l)a. It is pointed out that this 

 cannot hold for thick prisms, as in that case the distance 

 traversed by the light depends on the direction of entry, and 

 it is not justifiable to take the angles as being proportional to 

 the bases. 



