INTENSITY EELATIONS IN THE SPECTRUM OF HELIUM. 
149 
In connection with the interpretation of this table, it is necessary to remark, in the 
first place, that the actual numbers themselves give no information in the absolute 
sense, or in the relative sense down one column, as to the relative intensities of the 
three lines in question, for the photographic plate is not equally sensitive in three 
regions. But the actual changes in the numbers from one column to another give 
decisive information, since the intensity of A5876 is reduced to a uniform scale. 
These changes are very small, though quite definite, even taking into consideration 
the fact that the numbers are derived from an exponential type of foiinula, and 
they cannot be regarded as within the error of observation and consequent calculation. 
To at least a close approximation, however, the relative inteyisities in the Diffuse 
series do not vary with the distance from the cathode. 
The small variations which do occur present no striking regularity, and it is evident 
that the behaviour of the last three photographs, already stated to be on a different 
plate to the others, is not appreciably different in these regions, so that we have 
further justification for the supposition that the two sets of photographs are directly 
comparable. There is a small amount of evidence in the table, although it is not 
decisive, that a slight energy transfer to the longer wave-lengths takes place with 
increasing distance from the cathode, but if it be real, it is yet so small as to be a 
comparatively unimportant phenomenon. There is no effective transfer of energy 
along the Diffuse series of Helium with increasing cathode distance. 
Small variations in the numbers are to be expected, for it is difficult to maintain 
complete uniformity in the experimental conditions over a long period, and the 
various photographs were necessarily taken at different instants. But such variations 
in the conditions from one photograph to another apply to all the series alike, and 
from the uniformity of the numbers in Table V. we may assume with confidence that 
they are small. 
According to this conclusion regarding the absence of an energy transfer along the 
series, it is not difficult to show that the vanishing of \4026 on VIII. is to be 
expected. For on the basis of 10 for the photographic intensity of X5876, the 
average value for \4026 is 3'0. The actual photographic intensity of \5876 on VIII. 
is, from a preceding table (Table III.) 2’67. That of A4026 .should therefore be, on 
this scale, 
2-67 X 3-0/10 = 0-80. 
Accordingly, for this line, if h^ be its height, 
lOAV”' = 0-80 
which is less than unity, and therefore h is negative. This signifies that the 
exposure is insufficient to show the line even on theoretical grounds. In fact, on the 
scale in Tables III. and IV. the minimum photographic intensity which can be visible 
is not zero but unity. This particular scale, according to the definition of 
