Hacxert— The Photometry of N-Rays. 135 
minimum change he could detect was measured. This was done by finding the 
least distance it was necessary to move the screen forward in order that a change 
could be clearly perceived. In the case of this experiment the minimum change 
detected was 3 per cent., so that the heat effect of water at 20° must be less than 
this. 
The effects due to strained substances, then, can scarcely be assigned to heat, as 
water at 20° certainly did not cause a greater increase than 3 per cent., and the 
increase of brightness produced by water at 60 per cent. was much smaller than 
most of the effects due to strained substances. As the observations are free from 
subjective variations, we can only assign the effects due to substances under strain 
to some other objective cause than heat. The effects look large on paper, but in 
reality are not so. An ordinary person without any previous training found great 
difficulty in detecting small changes in this manner. The screen never seemed to 
be constant in appearance. 
These subjective variations obscured real changes of brightness, and their effect 
is clearly perceived in the minimum change which could be detected. The most 
common minimum for an untrained observer was about 10 per cent.; with a little 
more practice the minimum might be brought down to 6 per cent. Using the 
screen alone without any diaphragm raised the minimum change which was visible. 
It is evident that the effects described above would be invisible to a person whose 
minimum was 10 per cent., while a person whose minimum was 6 per cent. should 
see the effect, but in this latter case the increase of brightness is so close to the 
minimum that it would be difficult for the observer to definitely assure himself 
of any increase at all. It is not surprising, then, that so many have failed 
to see the effects. The possibility of observing a small change depends on training 
which enables one to get rid of subjective variations, and keep the appearance 
of the object constant, so that a small change in brightness can be observed. I 
wish to add that it was only after much practice that Mr. Quinlan and myself were 
able to assure ourselves of the changes I have mentioned. 
The reason why so many have failed to confirm Blondlot’s experiments is that 
the effects to be observed were only just within the range of visibility of all except 
those of considerable experience. The scientists in France who have confirmed 
Blondlot’s work are mostly those within his own circle, and others who have done 
work on photometry before, and consequently have got the necessary experience. 
This state of affairs is not satisfactory. As the existence of N-rays is only 
shown by their property of increasing the brightness of a phosphorescent screen, 
there is great need of knowing how this increase may be made manifest to any 
person without trouble, and in such a way that the effect can only be assigned to 
_ N-rays. 
TRANS. ROY. DUB. SOC., N.S., VOL. VIII., PART X. 2A 
