586 



Dr. Gr. Johnstone Stoney on 



Image 



image of the landscape will then be seen near the back of the 



objective. The light which forms this image consists o£ 



conical beams each with its vertex at some punctnm of the 



distant landscape and its base on the tiny front lens of the 



objective. If the experiment is made with a high-power 



objective, it is well to observe 



how exceedingly small that front Fig. 11. 



lens is. From this it is obvious 



that the light that enters the 



objective consists of beams which 



are very nearly parallel, or in 



other words of beams each of 



which is part of an undulation 



of waves that differ very little 



from being flat ; and each punc- 



tum of the image that is formed 



may be regarded as the concen- 

 trated light of one of these 



beams. 



The same succession of events 



takes place when the microscope 



is in use. In the air or oil 



space which lies in front of the 



objective, the light that is ad- 

 vancing upward may be resolved 

 into its component ufw's ; and 

 the portions of these that are 

 transmitted by the objective 

 undergo the same treatment as 

 the beams of light that enter 

 a telescope from a celestial 

 object. Each of them is by the Concentration I 

 objective brought to a focus in a image. \ 

 plane, or quasi-plane, perpen- 

 dicular to the optic axis and 

 near to the back lens of the 

 objective. There it forms the 

 kind of image which Airy in- 

 vestigated as the image of a 

 star, consisting of a central Image C -* 



" spurious disk " of light sur- 

 rounded by appendage rings. But little of the light is 

 expended in forming the appendage rings, so that we 

 may, as a first and usually as a sufficient approximation, 

 regard the whole of the light of each u f w as concentrated 

 into the central speck. The number of u f w's is unlimited, 



J 



