Pp ley 4 
xe 
THE PHOTOMETRY OF N-RAYS. 
By F. KE, HACKETT, B.Sc. 
[COMMUNICATED BY PROFESSOR J. A. M‘CLELLAND, M.A., F.R.U.1. | 
| Reap, May 17, 1904. | 
Jl, 
Ir is rare in physical science to find a divergence of opinion on a matter which 
ean be subjected to experiment. Perhaps never before was such a difference of 
opinion more justifiable than in the case of N-rays. The repeated failures to 
observe any of their effects seemed to totally disprove their objective existence. 
Since the observation of a change in dimly-illuminated objects is at present the 
only means of detecting the N-rays, it is important to know why the effect 
described by Blondlot is invisible to so many observers. 
This paper is an attempt to give the explanation, and to point out how the 
causes which affect such observations may be minimised. 
The first part of the paper gives some measurements of the subjective effects 
of the dark-adapted eye. This, it is hoped, will dispel some of the vagueness 
which hangs about these effects. The second part deals more particularly with 
the N-rays themselves. As most observers seem to have used screens of 
calcium sulphide made phosphorescent by daylight, it has been thought useful to 
give some measurements of the effect under these conditions. ‘These show that 
the effects are so small that they cannot be observed without special training. 
The conclusion of the paper describes the manner in which any person can see 
the effect without trouble, and test the objective existence of N-rays. The 
method of comparison is used, and the screens must be very dim. Some estima- 
tions of the effect under these conditions are given. No measurements are given 
except those which can be done with a certain amount of precision. The experi- 
ments have been carefully repeated after intervals ranging from one to two 
months, with concordant results. 
Before proceeding to the actual experiments, it is desirable to discuss the 
special point of view adopted in this paper regarding the observation of brightness 
with the dark-adapted eye. The main use of a phosphorescent screen is simply 
to afford us very conveniently a dimly-lighted surface. It is necessary in the 
TRANS. ROY. DUB. S0C., N.S., VOLe VIII, PART X. Z 
