150 
DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 
photographic intensity adopted is the true scale, and will be referred to as such in 
later parts of this conmiiinication. 
We may now consider Table VI. which shows that the Diffuse series of Parhelium 
behaves in the present connection in a quite different manner. The arbitrary 
intensity 10 is ascribed to X6678 in each case. 
Table VI.—Diffuse Series of Parhelium. 
Inteiisitv 
A. 
Photo¬ 
graph 
I. at 
cathode. 
Photo¬ 
graph 
II. at 
1 mm. 
Photo¬ 
graph 
III. at 
2 mm. 
Photo¬ 
graph 
IV. at 
3 mm. 
Photo¬ 
graph 
V. at 
4 mm. 
Photo¬ 
graph 
VI. at 
5 mm. 
Photo¬ 
graph 
VII. at 
6 mm. 
Photo¬ 
graph 
VIII. at 
7 mm. 
6678 
10 
10 
10 
10 
10 
10 
10 
absent 
4922 
5 ■ 3 
4-2 
4-2 
3-9 
4-6 
4-8 
absent 
absent 
4388 
5-0 
2-9 
3 ■ 3 
4-0 
6-3 
5-5 
7-3 
absent 
4144 
absent 
1-0 
10 
just seen 
absent 
absent 
absent 
absent 
There is an initial drop of Intensity on II. down this series, more pronounced in 
the third member, which becomes weaker relatively to the second. The fact that 
X4144 is not visible on I. is readily interpreted, for on this photograph, the true 
photographic intensity of A6678 is 6’25, so that A4144 would become invisible if its 
intensity on the arbitrary scale of the last table were less than 16, Avhich it may 
readily be, by comparison with the remainder of the last table. 
After this initial drop, a remarkable enhancement takes place in A4388, which is 
not confined to the last three photographs, and which cannot therefore be interpreted 
as due to a difference in behaviour of the separate plate on which they were taken. 
The change is of a quite different order of magnitude from any change found in 
Table V. 
On the apparent law followed by the rest of the table, the true intensity of A4144 
on IV. is found to be 1’2, which is in accordance with the fact that it is just visible. 
The intensities of this line on later photographs, even on the supposition of quite 
considerable enhancement after the manner of A4388, are all less than unity, so that 
its disappearance is to be expected. The theoretical intensity of A4922 on VII., on 
the same supposition, cannot exceed about 0’9 on the true scale, which is in accordance 
with its disappearance from this photograph. It is therefore true in general that 
the absence of lines in this series on the various photographs presents no disturbing 
-feature. 
The general conclusion regarding the Diffuse series of Parhelium is that, after an 
initial tendency to enhancement of the first member A6678 at the expense of the 
others, taking place almost exactly at the extremity of the dark space, there is a 
subsequent transfer of energy of the series to the higher members, as the distance 
