$34 Hacxertr— The Photometry of N-Rays. 
the image falls, by closing the eyes or otherwise ; (2) by any muscular agitation 
of the eyes. These changes take some time to die away, and, it can be ascertained, 
they are not due to a mere displacement on the retina. They do not seem to be 
present, as already noted, when the image falls within the fovea centralis. In 
some tentative measurements they amounted to about 7 per cent. Such effects 
depend so much on the observer and his state of health that it is not deemed 
advisable to do more than mention their existence until they have been tested 
under all kinds of conditions. 
II. 
The effect described by Blondlot could be observed by this screen and 
diaphragm in such a way that subjective variations were eliminated. Substances 
under strain were found the most suitable sources of N-rays. Wood or cork com- 
pressed, tempered steel, and unannealed glass were used for this purpose. As 
other more intense sources emit heat in addition to N-rays, their use could not be 
satisfactory. 
Some estimations of the magnitude of the effect perceived were taken. The 
screen and diaphragm were placed at such a distance that the pattern was rather 
indistinct. When a source of N-rays was brought up behind the screen, the 
pattern came out more clearly. The observer now noted the appearance of the 
screen. Whenit had returned to its initial condition after the removal of the N-rays, 
it was moved towards the observer until the decrease in distance had produced 
a change in the screen similar to that produced by the strained substance. The 
two positions of the screen were marked and afterwards measured, and the 
200D 
a 
The numbers obtained from different sources in this way for the same screen 
are given below. 
increase of brightness deduced as before from the formula 
Source. Increase of brightness. 
Unannealed Glass, J : . 10 per cent. 
Compressed Cork, ; ; : : D 96 
Sounding Tuning-Fork, . : : : Sia 
Silent Tuning-Fork, F : : : Spe ee 
Some idea of the effect of heat on the screen was got by using as a source of 
heat a column of water 5 em. long in a test tube. When the water was about 
60° C. it caused an increase of brightness of 5 per cent., while with water at 20°C. 
there was no noticeable change in the screen. In order to test the observer, the 
