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



hemispherical waves ; and at the same time the opening is a 

 large one when compared with the transversal* of the light 

 waves, since molecular considerations indicate that this trans- 

 versal (or rather these transversals, since there are two, an 

 electric and a magnetic one at right angles to one another) 

 must be regarded as of a length which is from the thousandth 

 to the ten thousandth of a wave-length. Hence the directions 

 of transversals will not be affected in passing through the 

 opening. On this account, if the incident light be a beam 

 of plane waves, whether polarized or not, the intensity of the 

 light will differ on the various parts of the hemispherical 

 waves which spread beyond the screen, being a maximum in 

 the direction of the prolongation of the normal to the in- 

 cident waves. This must be taken into account in attempts 

 to apply Airy's method of investigation to microscopic vision, 

 since until this is sufficiently done the investigation is too 

 imperfect for us to be justified in relying on its results except 

 so far as they can be confirmed by Abbe's method or some 

 other which does not involve the above consideration. A 

 further and more serious imperfection is introduced when 

 Airy's method is applied only to the light between the 

 objective and the image, and not also to the light between 

 the object and the objective. An inquiry conducted in this 

 way begins too late, after the more important of the events 

 that affect the image have occurred. Nevertheless it seems 

 to be the- only one which has as yet been made by Airy's 

 method ; see, for example, the investigation on p. 176 of 

 Lord Rayleiglr's paper. We shall learn in the second part 

 of this memoir what it is that in this case is being ascertained. 

 (7>) The other term in paragraph 1 that requires definition 

 is the objective field. By this term is to be understood the 

 whole of the object and its surroundings of w T hich an image 

 is formed by the telescope or microscope, or in the eye of the 

 observer. Accordingly the objective field at and surrounding 

 the object corresponds to ' the field of view 3 at and surround- 

 ing the image of it which is formed in the eye, or at the 

 focus of an optical instrument. 



* The word transversal is here and elsewhere used for the transversal 

 of the displacement under the dynamical wave theory of light. 



The dynamical wave theory is that used throughout this memoir, 

 except where otherwise stated ; since, in the present state of our know- 

 ledge, it is more easily handled than the electromagnetic wave theorv, 

 and since, except in .special cases (as for example the distribution of 

 intensity over a spherical wave), it furnishes the same results. Besides, 

 the dynamical theory usually carries us as far as we can go, for, in the 

 special cases where the electromagnetic theory may yield a different 

 result, it seldom happens that we yet know that result. 



