594 Dr. G. Johnstone Stoney on 



which the light between the prism and the telescope may be 

 resolved — or the light between the diffraction-orating and the 

 telescope, if the spectroscope be one furnished with a diffrac- 

 tion grating instead of a prism. 



So, again, the images seen in astronomical telescopes are 

 of the same nature as concentration images, since each visible 

 punctum of the image is the concentrated light of a beam of 

 parallel light. 



Aided by the information given to the microscopist by the 

 concentration image, it becomes possible for him to make 

 many interesting experiments and to understand them. These 

 experiments when interpreted bring to light many instructive 

 facts. For example, they will show that the image of an 

 extended object produced by a lens is not — as has been too 

 often supposed — the aggregate of the brightnesses of the 

 spurious disks with attendant rings, of its several puncta ; they 

 will show that the resolvability of two puncta upon a micro- 

 scopical object depends not only upon the interval between 

 those puncta but also in large part upon the detail present else- 

 where upon the object ; they will further show that there are 

 different kinds of resolution, and will indicate the conditions 

 which will result in resolution of the different orders. Other 

 experiments explain the obscure phenomenon of optical con- 

 tact, or exhibit instances in which colour can be introduced 

 into an image by excluding light of that colour from the 

 objective, and so on. To describe and interpret some of 

 these experiments must be our next task. 



Appendix (see p. 578). 



Any disturbance which can be propagated by a uniform 

 transparent* medium can be resolved into components each 

 of which is an undulation either of flat wavelets, or of spherical 

 wavelets which may be either concave or convex. And the 

 relation in which these three resolutions stand to one another 

 can be traced. 



47. To prove this, it is only necessary to recall the succesive 

 steps of the proof of Theorem I., which are given on pp. 571 

 and 572 of the B.A. Report for 1901, which will need but 

 little modification for our present purpose. To make the 

 process clear we shall exhibit the successive steps in an 

 individual instance. We shall select as the disturbance to be 

 resolved that which takes place within a given glass lens 

 while light emanating from external objects is traversing it. 



* By a transparent medium is meant one through which waves are 

 propagated without loss of energy. 



