570 REPORT— 1901. 



the gratuitous assumption of our ancestors that microscopical vision is an imitation 

 of macroscopical, and to become familiar with the idea that it is a thin^ sui 

 generis.'' 



This view has since been somewhat toned down ; but he still maintains that, 

 in the case of such an object as a diatom, there is practically a superposition of 

 two images, one depicting the coarse outlines and the other the fine details.' 



It is of courst' legitimate to mentally divide phenomena into two classes for 

 convenience of treatment ; but Huygens' principle applies equally to the fine and 

 the coarse parts of an object; and there is no way of obtaining true representation 

 of fine details; except by giving smallness to the discs which are the images of 

 points, seeing that the whole image, coarse and fine parts alike, is built up of these 

 discs. 



An important point, which is merely presented as an empirical fact in books on 

 the microscope, is the enormous benefit derived, in fine work, from employing a 

 sub-stage condenser of high quality to throw upon the object the sharpest possible 

 achromatic image of a limited portion of the source of illumination, an iris dia- 

 phragm, close to the condenser, being employed to assist in the limitation. The 

 reason of the benefit is that the influence of large aperture in reducing the size of 

 the discs which build up the image depends on the capability of mutual inter- 

 ference between all points of a wave-surface sent by a point of the object to the 

 focus. Two distant portions of the surface cannot interfere, if they are derived 

 from distinctly diflerent parts of the source of illumination. For purposes of 

 resolution, aperture counts only so far as it receives illumination from one and 

 the same source. If the four quadrants of an aperture are illuminated by four 

 separate sources, they will give, instead of a single small round spot, four larger 

 spots partially overlapping. 



A subsidiary benefit conferred by accurate focussing of the source on the 

 object is the prevention of the spurious patterns which are formed by the inter- 

 ference of light sent from a single point of the source to diflerent markings on an 

 object. 



7. On tlie Interference of Light Jroin Independent Sources, 

 By G. Johnstone'Stoney, M.A., B.Sc, F.R.S. 



In the course of an inquiry into the distribution of light by visible objects the 

 fact has emerged that lights from independent sources can be made to interfere, 

 whatever be their phases and states of polarisation. 



The present abstract is in reference to this point. To make it sufficiently brief, 

 it is limited to explaining the method of proof and giving one application to a 

 case which is easily dealt with, and where the result can be verified experimentally. 



The investigation starts from the admitted fact that in a transparent isotropic 

 medium the undulation spreading outwards from each punctum, or visible point^is 

 a train of waves of alternating electro-magnetic stresses of which the wave-fronts are 

 surfaces that are nearly spheres, or portions of spheres, concentric with the 

 punctum, and enlarging with the speed of light in the medium. 



Electro-magnetic stresses require an expenditure of energy to produce them or 

 to alter them, and in other respects there are analogies between the electrical 

 events with which we shall have to deal and dynamical events. Accordingly, as 

 we have a fuller nomenclature of dynamical than of electrical events it will be 

 convenient to speak of changes of electro-magnetic stress as motions in the medium, 

 of the cause of an alteration of the rate of change as a force, and so on, for this 

 purpose employing these and other dynamical terms in a sufficiently generalised 

 sense. 



We shall also have to assume that it is legitimate to apply the principle of 

 reversal to electrical as to dynamical events. 



Let us take a definite case, and suppose that P, a punctum or small source of 

 l)ght, is situated at a point /in the open {ether, from which it radiates light of wave 



' Carj>enter on the Microscoj>e, 8th edition, p. 64. 



