JOURNAL 



OF THE 



ROYAL MICROSCOPICAL SOCIETY. 



AUGUST, 1905. 



TRANSACTIONS OF THE SOCIETY. 



V. — On the Application of the Undulatory Theory to Optical 



Problems. 



By A. E. Conrady, F.R.A.S., F.E.M.S. 



(Read April 19th, 1905.) 



The discussions which have arisen over recent theoretical papers 

 seem to show that the simple applications of the undulatory theory 

 which have to b« called into requisition in the explanation of 

 optical images are not so well and so generally known as they 

 deserve to be ; and as it seriously disturbs the continuity of a 

 paper and leads to tiresome digressions if explanations of this kind 

 have to be interspersed, a short collection of the principal facts 

 may be acceptable and useful for future reference, and may also 

 assist many in understanding and appreciating the papers and dis- 

 cussions referred to. 



Whenever we try to get the utmost resplution out of any optical 

 instrument by increasing the magnifying power beyond a certain 

 moderate limit, we are confronted with facts which run counter to 

 the theories of geometrical optics, and which can only be accounted 

 for by taking into consideration the undulatory nature of light, i.e. 

 by rejecting the fiction of geometrical optics according to which 

 light consists of infinitely thin rays which can be united in points, 

 and by applying instead the principle of interference to these 

 problems. 



It has been proved by direct experiment that light travels at a 

 finite — though very great — speed ; the phenomena of interference 

 prove light to be of an undulatory or periodic nature, and the 

 further phenomena of polarisation force us to assume that the 

 vibrations are transverse ones, i.e. at right angles to the line of 

 Aug. 16th, 1005 2 E 



