220 Williams.—Observations on the Action of 
A range of intensities is possible if one places the slide at different distances 
from the bulb, since the intensities then fall off inversely as the squares of 
the distances. 
4. In following the series of changes strips were removed from the slide 
after various times of exposure and examined both with the high power 
and with the paraboloid form of ultra-microscope. 
The first change seen was an increase in the rate of circulation, of 
a type which must be defined clearly. In this material there is no distinct 
movement of rows of chloroplasts as in some plants, but circulation of the 
protoplasm is shown by the movement of bright particles, which focusing 
shows to be near the wall, along lines parallel to the long axes of the cells, 
and a similar movement along protoplasmic threads leading to the centre of 
the cell. In all discussion of circulation only bright particles of similar size 
and appearance, moving along lines parallel to the long walls, are con¬ 
sidered. 
As exposure proceeded there was an increase in the amount of 
Brownian movement executed by these particles. Since the temperature 
was steady and the sizes and optical properties of the particles remained 
constant, this increase in movement would appear to denote a fall in the 
viscosity of the medium. As there was a forward motion imposed upon the 
particles by the movement of the protoplasm it appeared as though relative 
values of the viscosity at different stages could be obtained by observations 
on the curves executed by the particles, according to the method used by 
Svedberg ( 5 ). Unfortunately the amplitudes of the vibrations were too 
small to make such estimations possible. 
Longer exposure to the rays caused a diminution of circulatory rate, 
and finally cessation. 
It was hoped to obtain figures of quantitative value by plotting graphs 
of circulatory rates after various times of exposure, but the movement in the 
plant cells proved to be influenced by too many factors to make success 
likely, as the nature of the tests did not allow time to compare the specimen 
with its own control at each stage. 
Measurements were made to find whether the stimulation indicated by 
increase in circulatory rate was followed by a depression when treatment 
ceased before there was any outward sign of injury in the cells. For each 
of these tests strips were divided longitudinally ; half was exposed and half 
kept as a ‘ control ’. Circulatory rate was measured in each at intervals— 
readings being taken after ten minutes’ exposure to the light of the micro¬ 
scope mirror, and measurements being made as quickly as possible on 
central cells in the strip. 
The table below shows some representative experiments. In no case 
was the exposure long enough to produce visible injury either after ex¬ 
posure or at the end of twenty-four hours. 
