Hacxrert—The Photometry of N-Rays. 129 
* which appeared to an observer exactly the same as the diaphragm and screen 
described above, and varied in the same way. The distance at which the middle 
black line vanished for a given distance of the gas flame from the screen was 
determined. If this law be true, then the product of these two distances should 
be constant. It was found that this condition was roughly satisfied, so that it 
might be used in getting some general idea of changes in brightness in the 
manner adopted in this paper. 
Many measurements were made on this assumption. Such measurements are 
necessarily very rough. They are not intended to be quantitative, but only to 
give some idea of the change of brightness observed. It isa great obstacle in 
experiments in N-rays that one does not know what change to expect. It is 
hoped some of the numbers given may be of use in forming an estimate of the 
changes in brightness which take place. 
The considerable variation of sensitiveness over the retina when the eyes are 
being used in darkness is well known. Lummer, in an address to the German 
Physical Society,* pointed out the great necessity of paying attention to this 
property. It was found necessary, at a very early stage in this investigation, to 
get more definite information on this and other peculiarities of the dark-adapted 
eye, than the vague statements which are given, even by the best authorities on 
the physiology of the eye. This property of the retina was investigated by 
drawing lines of equi-sensitiveness. Captain (now Sir William) Abney has given 
such curves in his paper entitled ‘‘'The Sensitiveness of the Retina to Light and 
Colour,”+ but these, however, were only got for the external parts of the retina 
in daylight. He suggested the name ‘isolumes” for these curves. 
The isolumes for the dark-adapted retina cannot be expected to have any 
relation to those got in daylight. It was found quite simple to take observations 
around and within the yellow spot. They were made in a manner which is the 
same in principle as Abney’s method, and require some trouble and patience. 
In applying this method, the object used was the combination of diaphragm and 
screen already described. The slits are, however, of such a size that when they 
appear blurred their image on the retina is not larger than the fovea centralis, 
which is a circular area3 mm. in diameter. A bright spot of light (some calcium 
sulphide at the end of a rod)is used to fix the position of the eye. When the eye 
is looking at the bright spot, its image falls on the fovea. In the experiment the 
eye was made to follow the spot of light as it was moved horizontally across the 
diaphragm away from the slit. As the distance between the spot of light and the 
slits increased, the latter became more and more distinct, until the black line 
* Nature, Feb. 18th, 1904. } Phil. Trans., vol, exe., 1897, A, p. 155. 
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