130 Hacxett—TZhe Photometry of N-Rays. 
could be clearly distinguished. The position of the bright spot of light 
was then marked. The process was repeated on the other side of the slits. 
The spot of light was moved, until apparently the slits came out just as clearly. 
The same thing was done above and below the slits. These positions could 
be clearly determined, as the change of brightness, though slow at first, 
increases rapidly afterwards. From these readings we could take a plan of the 
retina, and plot a curve in it which would be a line of equi-sensitiveness. 
Another observation yet remains to be mentioned. After the four positions had 
been determined, the screen was moved forward, until apparently the slits came out 
as clearly as before. Assuming the law of inverse squares, the apparent increase 
of brightness for small changes is equal to twice the distance moved forward, 
200D 
Ed 
The measurement of brightness in this way is open to much objection. But the 
percentages may be regarded in another light, which is equally useful. The 
numbers given are twice the percentages of the changes made in distance, in 
order to produce the equivalent changes in distinctness. From this point of 
view the measurements are independent of any assumption, and comparisons can 
at BID 
divided by the total distance; in symbols PR? reduced to percentages, 
be made among them. 
; : ID. 
In accordance with this the formula ale will be used in every case, though 
it is inaccurate for measuring increases of brightness when large changes take 
place. 
By means of the curves drawn from these observations, we can form some 
idea of the behaviour of the retina around the yellow spot. It is only this 
region which is of importance in this subject. 
It is desirable that observations of this class should be made with more than 
one individual. For this reason, two series of observations have been made—one 
by Mr. Quinlan, and the other by myself. And here I desire to express my 
obligations to Mr. Quinlan, who has been of much service to me in the course 
of the investigation, in enabling me to test and confirm observations again and 
again. 
It has been thought sufficient merely to give those curves which map out the 
gensitiveness of the retina over the area observed. It is well to mention that 
observations taken with an interval of two months were concordant. It is 
remarkable with how much precision readings can be taken: the curves are drawn 
directly from observations, but it was only after some practice that such precision 
was acquired. Tig. 1 represents Mr. Quinlan’s observations ; fig. 2, my own. 
The numbers on the curves are a rough indication of the change of 
