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



turbed. These are both of them phenomena more conspicuous 

 in others of the experiments which we propose to make, and 

 will more appropriately be explained in connexion with them. 

 (See § 37.) 



It has appeared desirable to go very minutely through the 

 successive steps of this example in order to show clearly that 

 Abbe's mode of dealing with microscopic vision does success- 

 fully and in the most instructive manner guide every step 

 of our preparations, and that it renders a full explanation of 

 every phenomenon we encounter whether in the course of 

 the preparations or in the final result. 



36. On the Significance of ivhat ive see in the Microscope. — 

 As to the significance of these specks. Their spacing along 

 each row is somewhat less than the spacing of the rows, and 

 may be taken to be about 0*24 fi. From the way in which 

 their image is formed by the intersection of rulings of the 

 first order, it appears that they, like rulings of this kind, will 

 appear larger or smaller according to the amount of the 

 illumination ; and that they will be best seen when the 

 illumination is such that the apparent diameter of a speck, 

 and the apparent interval between it and the next, are about 

 equal. Accordingly the portion of the object which corre- 

 sponds to it when best seen is that which is contained within 

 a sphere whose diameter is 0'12/jl, and whose radius is there- 

 fore 0'06fjL. Now if we make the probable hypothesis (see 

 " On the Internal Motions of Gases/' § 9, in the Phil. Mag. 

 for August 1868, p. 140) that the average spacing in solids 

 and liquids of the chemical atoms of w'hich matter is made up 

 is somewhere about a tenthet-metre*, which is the same as 

 0-0001/A, then there has needed about 900,000,000 of these 

 chemical atoms to build up a portion of the object of the 

 volume of our little globe. If, on the other hand, we suppose 

 that the spacing of the atoms is about a ninthet-metre, i. e. 

 0'001/z, — and it can hardly be more than this — the number of 

 atoms in our little sphere becomes 900,000. The actual 

 number probably lies somewhere between 900 thousands and 

 900 millions. There is therefore this vast number of chemical 

 atoms in the tiniest part of an object which can be dis- 

 tinguished from another part by so good a microscope, when 

 handled with extreme care. We ought further to reflect that 

 each of these chemical atoms is itself highly complex, and 

 that within every one of them all those events are in progress 



* A tenthet means a unit in the tenth place of decimals. And as a 

 quarter pound means the quarter of a pound, so a tenthet-metre means 

 the tenthet of a metre. 



