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



the rearrangement of particles, the viscosity steadily lessens, 

 this entitles us to suppose that with an alternate magnetization 

 the maximum of it does not reach that quantity attained by a 

 static process. And certainly the most recent observations on 

 the exhaustion of iron in transformers seem to corroborate this. 

 From the point of view of the present hypothesis, the exhaus- 

 tion of iron is nothing but the rearrangement of its particles, 

 in consequence of which the magnetization is performed with 

 a smaller absorption of energy, and therefore calls for a less 

 pronounced magnetic effect- 

 Physico-chemical Society. 

 The University, St. Petersburg, Russia. 

 June 15, 1896. 



XXXV. Microscopic Vision. By G. Johnstone Stoney, 

 M.A., D.Sc, F.R.S* 



Part I. — Fundamental Principles f . 



1. "YTISION, whether by the naked eye or with the as- 

 V sistance of optical instruments, may be studied in 

 many ways ; since a correct investigation may start from any 

 one of the innumerable possible resolutions of the disturbance 

 which exists throughout the aether in front of and close to the 

 object. But two only of these will be here considered, viz.: 

 that most obvious resolution in which the sether in front of 

 the object is regarded as traversed by undulations of hemi- 

 spherical ivaves emanating from each physical point of the 

 surface of the object, and that other equally general but less 

 obvious resolution of the disturbance in this portion of the 

 aether into undulations of uniform plane ivaves transmitted 

 forwards in all or some directions from the whole extent of 

 the objective field. 



2. The first of these modes of resolution — that into sphe- 

 rical waves — is the foundation of Airy's method of studying 

 the images formed by telescopes, in which the image is re- 

 garded as arising from the overlapping and interference of 

 the spurious disks with attendant rings which in the image 

 take the place of points on the object. This method has on 

 this account been sometimes called the Spurious Disk Theory. 

 The second mode of resolution — that into plane waves — is the 

 foundation of Abbe's method of studying the images formed 



* Communicated by the Author. 



t Part II. deals with the application of these principles to the micro- 

 scope as at present made. 



