598 Chronicles of Sctence. [ Oct., 
VIII. PHYSICS. 
Liaut.—Professor Roscoe, of Owens College, writes us that in the 
last number of our Journal, in an article on De la Rue and Celestial 
Photography, the author has unwittingly done him an injustice in 
attributing to him the claiming of a discovery which was already 
known. Referring to the statement as to the well-known difference 
between the intensity of light from the centre and from the border 
of the sun’s disc, Dr. Roscoe writes, “‘ What I believe to be original 
in my communication, is the nwmerical determination of the amount 
of a difference which has been long observed.” 
Dr. Memorsky and Professor Brucke, in a paper communicated 
to the Vienna Academy, describe diffuse daylight as strongly 
reddish, just as gas ‘or lamplight is yellow. The only perfectly 
white light, they tell us, is the electric light from charcoal points. 
The hght of burning magnesium, and the combustion of phosphorus 
in oxygen, they say are violet. 
M. A. Bertin has examined the constitution of glacier ice by 
polarized light. He has found that the superficial part of the 
higher glaciers is composed of agglomerated snow; but lower down, 
where the water has sunk into the fissures and become frozen, 
crystallization and true ice are found. 
Professor Bunsen has made a discovery in connection with the 
absorption-spectrum of Didymium which may prove of great 
theoretical importance. In a paper which he published in con- 
junction with Professor Bahr some time ago, it was shown that 
slight differences were observed in the absorption-spectrum of. 
sulphate of didymium according as the light was allowed to pass 
through a crystal or through a solution of the salt. Since that 
time Professor Bunsen has found that the erbum and didymium 
spectrum undergo alteration if polarized light be employed and 
either the ordinary or the extraordinary ray be allowed to pass 
through the crystal. It has also been found that whilst, when 
spectroscopes with one prism and with a telescope of moderate 
power are employed, the spectrum of the various didymium com- 
pounds do not show any difference, yet most undoubted differences 
are noticed when more powerful instruments are used. These 
differences cannot, in our present complete state of ignorance of any 
general theory for the absorption of light in absorptive media, be 
connected with other phenomena. They remind one of the slight 
and gradual alterations in pitch which the notes from a vibrating 
elastic rod undergo when the rod is weighted, or of the change of 
tone which an organ-pipe exhibits when the tube is lengthened. 
These curious phenomena form the subject of a long paper by 
