2 Rheinberg: Common Basis ofthe Theories ofMicroscopic Vision. XIX, 1. 



able paper on microscopic visiou by Mr. .T. W. Gordon ^ there is 

 likely to be a revival of iuterest and controversy on this important 

 subject. And secondly, because tlie metliod of showing and explainiug 

 the action of a dift'ractiou grating by snccessive stages , beginniug 

 witli two slots only, may possibly in itself be found somewhat novel, 

 and help to link togetlier apparently contiicting theories. 



Chapter I. 

 Elementary Consider at i ons. 



Two of the fundamental principles of the wave theory of light 

 are : firstly , that every luminous point is a centre of disturbance 

 from whieh waves are propagated in the ether, and secondly that 

 every illiiminated point likewise is , or may be considered , as a 

 similar centre of disturbance. 



It should be noted that the expression „ilhiminated points" is to 

 be taken in its widest sense , as indicating not only points on the 

 surface of any body on which the light falls, but also points in the 

 intermediate ether. It follows that every luminous point is surrounded 

 on some or on all sides by innumerable illuminated points , or in 

 other words, every primary centre of wave motion is surrounded by 

 innumerable secondary centres of wave motion to which it gives rise. 



The light at any given point is necessarily the resultant of the 

 undulations which arrive there from all the luminous points from 

 which waves cau reach it. The Import of the preceding paragraph 

 lies in the fact, that the light at the given point may be determined 

 not only by reference to the luminous sourees themselves, but — 

 given a knowledge of the State of wave motion on any intermediate 

 lines or curves between the point and the primary centres of distur- 

 bance — we can trace the nature of the light at the given point 

 by reference to these. 



The way in which this is done is to firstly find the effect due 

 at the given point to each point of the luminous source separately 

 — and afterwards to add tlieir Joint ert'ects. 



^) Gordon, J. W., An examination of the Abbe diftVaction theory of 

 the microscope (Journ. II. Microsc. Soc. 1901, pt. 4, p. 353, 475. See also 

 this Journ. vol. XYIII, 1901, p. 296). 



