Lonization produced in Gases by Réntgen Rays. 85 
In making this comparison it was very essential that the 
source of ionization should remain constant in intensity 
throughout the series of observations, otherwise no comparison 
could be made unless the amount of variation in the intensity 
was known. Even when using an automatic regulating bulb 
it is almost impossible to obtain perfect regularity in the 
intensity of the rays. When taking the observations the bulb 
was run as far as possible at regular intervals so as to keep 
it steady, and also several readings were taken in each case 
and the mean taken. However, even with these precautions, 
one cannot be certain as to whether the rays remain constant 
in intensity throughout the experiment unless some indepen- 
dent check is employed in order to test their constancy. For the 
purpose therefore of testing the constancy of the rays a small 
standard apparatus was introduced between the source of the 
rays and the large cylinder as shown in fig. 1. Itis shown in 
detail in fig. 2. It consisted simply of a rectangular cylinder 
Fig. 2. 

made of sheet-lead about 12°5 cms. square and 10°4 cms. in 
length, and the ends were covered with paper. It contained 
two parallel zinc plates, about 9 ems. square, acting as elec- 
trodes. One of these plates was connected to the same pole of 
the battery as E. From the central part of the other plate 
was cut a circular disk 3°5 cms. in diameter, and this was 
