84 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 



absorbings 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 indispensible 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 ouestion. 



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 more 

 centimeters on an edge, but at least so large that such a prism 

 could be buill up by cementing parts of it together. Experiments 

 had been made at the General Electric Co. for pi-oducing 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 

 degree of certainty could not be secured with the apparatus which 

 Mr. Aldrich and the director had designed and which Mr. Kramer, 



