160 Dr. G. Johnstone Stoney on Mr. Lewis Wright's 



that is, the motion in it is such that it travels forward, 

 and that its progress is controlled exclusively by the internal 

 forces of the medium. As soon as the motion reaches this 

 condition of independence of external agency, it becomes 

 resolvable either into the spherical waves employed by Airy, 

 which decrease in intensity as they advance, or into the 

 plane-wavelet components of my theorem, which advance 

 without change. Every deduction which can be worked out 

 by the former method of resolution can be worked out by the 

 latter; and several others also, on account of the less compli- 

 cation which the new method of resolution presents to the 

 mathematician. This is the reason, and the sole reason, of 

 the greater efficiency of the new method in dealing with 

 microscopic vision, the adequate investigation of which 

 presents more difficulty than the investigation of telescopic 

 vision. 



Again, Mr. Wright, on p. 483, imagines that the narrow- 

 ness of the chink between the object and the front of an 

 immersion objective, which, as he explains, may in an 

 extreme case be only the 200th of an inch (0*127 of a milli- 

 metre), makes it "impossible to regard light emitted from 

 an object as consisting of uniform plane waves on arriving 

 at the surface of such a lens," except when the illumination 

 is of a special kind, which he proceeds to describe. Now, 

 inasmuch as the turmoil spoken of in the last paragraph 

 does not extend to more than a wave-length or two from 

 the object, while the interval between object and objective, 

 as described by Mr. Wright, is about 300 wave-lengths of 

 red or 400 wave-lengths of blue light in the oil, glass, &c. 

 that are interposed, it follows that nearly all the width of 

 this interval lies beyond the region of turmoil and is occupied 

 by luminous undulations propagated solely by the internal 

 forces of the medium. All such undulations are identical 

 with the traversing of the same space by the trains of plane 

 wavelets of my theorem, one of these trains advancing in 

 every direction towards which a ray of light is directed. 



Moreover this is independent of how the object is illu- 

 minated ; so that Mr. Weight is also mistaken in regard to 

 the exception which he makes. 



Similarly, when Mr. Wright supposes that the source of 

 light illuminating a microscopic object must be distant, 

 or must transmit a pencil of light of small angle, in order 

 that it may consist of the plane wavelets of my theory, he has 

 quite misapprehended the subject. In all cases — whether the 

 cone of illumination be large or small, whether diaphragms 

 are used, whether there is a condenser, and whether the con- 



