138 Hacxert—The Photometry of N-Rays. 
can be observed under conditions which admit of no subjective or heat explana- 
tion. But I have not succeeded in observing many other peculiar properties of 
N-rays under such satisfactory conditions, with a single exception. In carrying 
out an experiment with dim screens, an initial decrease of brightness was re- 
marked in the screen under the influence of N-rays in certain cases, which was 
soon succeeded by an increase of brightness in the same screen. The effect 
corresponds to Blondlot’s N,-rays, but it does not lend itself to measurement, nor 
can its existence be rigorously tested, as it is only a temporary effect. 
In conclusion, I desire to express my great obligations to Professor M‘Clelland, 
who suggested I should inquire into this subject, and gave me much encourage- 
ment and advice during the progress of the investigation. I also wish to mention 
that I am greatly indebted to Professor Coffey for enlightening me on 
physiological questions. 
