150 



DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 



photographic intensity adopted is the trtie scale, and will be referred to as such in 

 later parts of this communication. 



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. 



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 X6678 is 6'25, so that X4144 would become invisible if its 

 intensity on the arbitrary scale of the last table were less than 16, which it may 

 readily be, by comparison with the remainder of the last table. 



After this initial drop, a remarkable enhancement takes place in X4388, 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 X4144 

 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 X4388, are all less than unity, so that 

 its disappearance is to be expected. The theoretical intensity of X4922 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 X6678 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 



