REPORT OF THE SECRETARY. 81 
range of wave length from 5 microns to 50 microns. There is no 
surface either of blackened metal or other substance which is fully 
absorbing to the rays throughout this whole extent, and further- 
more there is no optical medium known by means of which the prop- 
erties of the rays beyond about 17 microns, where rock salt ceases to 
be transparent, may be investigated. For the purpose of determin- 
ing nocturnal radiation it seems absolutely indispensable that there 
should be devised an instrument based upon the principle of the 
perfect radiator or " absolutely black body." This is a very difficult 
thing because not only does the instrument have to be exposed to 
the full hemisphere of 180° of solid angle, but also the radiation to 
be observed is small in amount, little more than the tenth part of 
the radiation of the sun. Seeing that the " black body," so called, 
requires to be a hollow chamber, large with respect to the aperture 
through which the rays enter, the rise of temperature of its walls 
which must be measured is extremely small. After much consulta- 
tion, Mr. Aldrich and the director decided upon a design of a new 
instrument for this purpose. This was constructed in the spring of 
1919, and is now in use on Mount Wilson. Whether it will prove to 
be satisfactory or not remains a question. 
In order to investigate the rays beyond the wave length where 
rock salt becomes opaque a great many measurements have been 
made by Mr. Aldrich, as mentioned in the last report, to attempt 
to find some substance transmissible to such rays. The best sub- 
stance found appeared to be potassium iodide. It usually occurs 
as crystals no larger than a buckshot. Accordingly, in order to 
make any satisfactory progress it was necessary to procure larger 
crystals, preferably large enough to make a prism of five or m^ore 
centimeters on an edge, but at least so large that such a prism 
could be built up by cementing parts of it together. Experiments 
had been made at the General Electric Co. for producing large 
crystals needed in war operations, and they very kindly undertook 
to try to grow potassium iodide crystals also. A number of crystals, 
very satisfactorily clear, have been produced by them as large as 
2 centimeters on each edge, and from a sufficient number of these the 
prism required for going on with this long wave length work may 
probably be formed. 
Mr. Aldrich spent a long time on the development and testing 
of an apparatus for determining the constant of the fourth power 
radiation formula ordinarily called a. This is a very difficult 
research. The quantity is already certainly known within 5 per 
cent and "many physicists would believe even closer than this. Many 
researches have been made upon it and in order to do a piece of 
work worth while it is necessary to show that it is certainly accurate 
to 1 per cent. After many experiments it was found that this 
