﻿642 Prof. Porter and Mr. Hedges on the Law of 



hauteurs inferieures an dixieme de millimetre, que Ton peut 

 etudier utilement les emulsions que j'ai employees. " 



We have thought it important not only to prove that the 

 concentration tends to uniformity as the depth increases, but 

 also to find the law of change when the concentration ceases 

 to be sufficiently dilute for the perfect gas equations to hold 

 good. 



Experimental. 



A suspension of gamboge was prepared by rubbing the 

 solid under distilled water with a soft brush. From this, 

 one in which the particles were practically of the same size 

 was obtained by means of fractional centrifuging. The 

 process actually adopted was that worked out and described 

 by E. Talbot Paris*. 



To count the number of particles at various depths, a 

 modified arrangement of the Zeiss ultra-microscope was 

 used. The cell in which the suspensoid was contained was 

 viewed by the microscope placed with its axis horizontal, 

 while it was illuminated by a vertical beam passing down- 

 wards through the cover slip which served as a lid to the 

 cell. This vertical beam was obtained from the horizontal 

 beam passing from the lantern through the usual lens system 

 by means of suitably placed reflexion prisms. 



The final condensing lens with its screw adjustment was 

 placed with its axis vertical, and the cell was attached to and 

 movable with this lens, so that various depths could be 

 examined in turn. An iris diaphragm in the eyepiece per- 

 mitted the field to be cut down until the number of particles 

 visible at any moment was sufficiently small to be estimated 

 at a glance. To diminish convective effects the light was 

 passed through a water-cell, and a shutter was employed so 

 that the light only passed through the observation-cell for 

 a time sufficient to enable the particles to be counted. With 

 the exterior of the cell blackened and all stray light screened 

 off, it was found possible to count the particles in the field of 

 view over a range of several millimetres from the surface. 

 This arrangement was not convenient for making obser- 

 vations so close to the surface as those made by Perrin 

 which were all confined within the range of less than 

 O'l mm. ; but this was no drawback to the object we had in 

 view. 



The method of obtaining the concentration at any depth 

 was to count the number of particles in the field of view 

 from twenty to forty times at intervals of ten seconds, and 

 * Phil. Mag. xxx., October 1915. 



