262 



Dr. G. W. Royston-Pigott. 



real objects could be detected by the microscope of the surprising 

 degree of attenuation represented by the millionth. 



For this purpose a long search was instituted for many months, both 

 for the finest attainable defining powers and for suitable objects. 

 Minute mercurial particles have appeared to furnish very interesting 

 evidence. If these be very finely smashed with a small piece of watch- 

 spring, the experiments will be promoted by inserting a minute drop 

 of petroleum beneath the "cover," which should be very thin and 

 clean (fig. 9). 



A reflex illuminator is then used to converge rays downwards 

 through the objective upon the preparation. 



The most extraordinary minute disks (in a darkened room) may be 

 then observed (of a circular form chiefly). Some of them are irregular, 

 and upon some of them will be detected clusters of the most wonderful 

 degree of smallness, consisting of minute black points, visible with a 

 power of 1,000 diameters. And what is so surprising is the entire 

 absence of any diffraction effects at the edges of these black points. 



For convenience of ready reference, I have found micrometer spider- 

 lines of service for comparative estimation. There are three thick- 

 nesses carefully measured : — 



No. 1 J- 5,000th of an inch. 



No. 2 1- 8,000th 



No. 3 l-10,000th 



Comparing these black particles floating on the surface of the mer- 

 cury with the apparent diameter of the thinnest spider-line, some of 

 them appeared decidedly smaller. This observation has been confirmed 

 by several persons to whom they were exhibited. 



Under 1,000 diameters the particle was magnified just one hundred 

 times in the micrometric focus. It then appeared less than the spider- 

 line 1 ooo o of an inch thick. Its real diameter was therefore less than 

 the of this magnitude, or less than the millionth of an inch, a 

 microscopic unit which I propose here to represent by Ul. Under a 

 power of 2,000 diameters the finest spider-line would represent the 

 quantity \ 111. 



The accompanying drawing fairly represents the appearance of the 

 spider-lines seen on a dark field when mercurial particles, as already 

 described, are examined with the micrometer under a power of 1,000 

 (Plate 3, fig. 9). (The webs appear too coarse in the plate.) 



A positive eye-piece utterly spoils the finest defining powers, and a 

 Huyghenian is generally substituted.* 



The very beautiful phenomena observed by means of brilliant illu- 

 mination of these minute mercurial disks, flattened by pressure or 



* Definition may be further improved by substituting a crossed lens for the field' 

 and an achromatic magnifier for the eye-lens. 



