MAPPING THE LEAST REFRANGIBLE END OF THE SOLAR SPECTRUM. 667 
The blue transmitting form of silver bromide can readily be transformed into the 
red transmitting form by simple friction, in which state it becomes as insensitive to 
the lower octave as if the molecule had been formed by chemical means, as by the 
ordinary methods of preparation. A difference in molecular state for other com¬ 
pound bodies has been suspected by Mr. J. Norman Lockyer,* and this existence 
of two states of silver bromide lends confirmation to his view. 
I have to thank the Astronomer Royal for kindly placing diffraction gratings at my 
disposal, as also Dr. J. W. Draper and Professor Langley for procuring me others, 
with which the most successful part of my work has been carried out. My assistant, 
Corporal Jackson, R.E., has been most invaluable to me in making the various emul¬ 
sions under my directions and in taking negatives when my official duties prevented 
me from personally utilising what little sun we have had during the last summer and 
autumn. To Mr. Dick my acknowledgments are also due for the care he has taken 
in preparing the map of the diffraction solar spectrum. 
* Proc. R. S., note 11, page 153, 1874 .—‘ Studies in Spectrum Analyses,’ page 120. 
