Dr. G. J. Stoney on Microscopic Vision. 339 



of the object in the objective field ; and, finally, if the travelling 

 backwards is continued long enough for the undulations to 

 reach the positions that had been occupied by the original 

 object, they will there produce an image of it the most perfect 

 which the light that had been emitted by the object is capable 

 of producing. This image thus becomes a standard of per- 

 fection which may be approached but cannot be exceeded by 

 the images formed by any optical contrivance from the same 

 light. 



From the way in which the standard image is formed it is 

 manifest that it is an image of the same size and general 

 shape as the object or group of objects represented by it. 

 The further excellence of this image depends upon the amount 

 of detail upon the object which it is competent to reproduce ; 

 and this varies, as we shall find presently, with the wave- 

 length of the light employed, and with the way in which that 

 light has been supplied to the object. The standard image 

 may be regarded either as viewed from beyond, or as being 

 transformed into a picture by being thrown on a screen able 

 to scatter whatever light falls on it. The screen should not 

 be flat, but with such prominences and depressions as will 

 enable it to catch the light everywhere exactly where the 

 image is formed. Such a picture is entitled to the name of 

 the standard picture, since it has on it all that part of the 

 detail on the real object which the light is capable of 

 showing*. 



9. Theorem 2. The Standard Image. — Let us consider 

 somewhat more closely how the standard image is formed. 

 It is formed by the coalescence and mutual interference of 

 uniform plane waves. Now when we consider how these 

 same undulations originated when starting on their outward 

 journey and remember that the condition of the aether is the 

 same on their return, except as regards the direction of 

 the motions ; when we further remember that the point p 



* Another way of conceiving the standard image which is for some 

 purposes more convenient, is to imagine the retreat of the luminous 

 undulations to be carried farther backwards (the condenser of the 

 microscope and any other obstruction being of course removed) : and 

 then, at a given instant, to conceive the setherial motions to be again 

 reversed. The undulations will thereupon travel forwards (i, e. in the 

 direction in which the light originally moved), will re-form the standard 

 image when they reach the position that had been occupied by the 

 object, and will thence proceed to the objective of the microscope in 

 precisely the same state as was the light that was transmitted to it by 

 the real object. It thus appears that the source of light, the condenser, 

 and the object may be all removed, and that the standard image emitting 

 its light forward may be substituted for them. 



2B2 



