424 Dr. G. J. Stoney on Microscopic Vision. 



hemispherical waves ; whereas, in consequence of molecular 

 events, the light really comes off in a broken succession of 

 such trains, each of moderate duration and, generally, discon- 

 tinuous from one train to the next. Within each train there 

 must be such continuity as is manifested by the prolonged 

 interference effects found in the use of such instruments as 

 Rowland's largest diffraction gratings, or Michelson's Refrac- 

 tometer. But, to bring about these effects, the average length 

 of the trains need not be more than some such length as a 

 metric foot (30 centimetres) which includes about 500,000 

 luminous waves from the brightest part of the spectrum. 

 This would correspond to an average duration of trains of 

 about the thousand-millionth part of a second. But however 

 brief the duration of each train, nevertheless for that short 

 time the setherial disturbance it occasions can be resolved in 

 whatever ways it could have been resolved if the train had 

 gone on for ever. Hence, in order to take into account the 

 discontinuity of the trains of waves as actually emitted by the 

 point pj it suffices simply to recognize that the undulations of 

 uniform plane waves of Proposition 1 are made up of a suc- 

 cession of comparatively short sections of uniform plane waves, 

 which may each include only some few thousand waves and 

 which may be discontinuous where one succeeds another. 

 This, however, does not hinder their continuing to be undula- 

 tions of uniform plane waves : accordingly Proposition 1 

 extends to the cases where the emission of light from the points 

 of the object is discontinuous* 



[ Corrections in Part I. — In the heading of § 8, p. 338, 

 change " Principles of Reversal " into Principle of Reversal. 

 In the footnote on p. 336, line 7, change " wave " into waves. 

 In the diagram on p. 337, the repetitions of the curve mn 

 should have been drawn of the same form as the original 

 curve.] 



Part II. 



21. The Illuminating Apparatus. — The state in which light 

 reaches that portion of a microscopic object which is under 

 scrutiny is determined by the source of light, by the con- 

 denser, by the use that is made of the iris diaphragm and 

 stops which are associated with the condenser, and by the 

 parts of the microscopic object through which the light has 

 to pass to reach the part which is being specially examined. 



The iris diaphragm and stops determine the directions from 

 which light is allowed to reach the object. These should be 

 such as will bring into sufficient prominence in the image 



