Rays of Great Wave-length in Rock-salt , Sylvite, fyc. 37 



The first change toward the accomplishment of this end 

 was effected in the substitution of two plane surfaces of larger 

 dimensions in place of the reflecting plates formerly used. 

 For this purpose the optical firm of Carl Zeis, of Jena, most 

 generously provided us with two plates with plane surfaces, 

 4 centim. square and 1 centim. thick, one of crown glass and 

 the other of fluorite. The plates were set in metal frames, 

 and the distance between them regulated by a system of 

 screws, as in the former case. With the exception of the 

 extreme edges both plates were ground to a truly plane sur- 

 face. A rectangular opening in a diaphragm, placed in the 

 path of the rays, allowed only light to enter the slit of the 

 spectroscope which had been reflected from the central portion 

 of the plates. The interference-bands thus produced were 

 unusually sharp, as can be seen from the pronounced minima 

 of the curves in figs. 1, 2, and 3 (Plate II.), which represent 

 the three different energy-spectra. Hardly need it be men- 

 tioned that in the following experiments the entire optical 

 system consisted wholly of rock-salt and fluorite. 



The delicacy of the bolometer was increased chiefly by using 

 a galvanometer of the highest degree of sensitiveness which one 

 of us had constructed, and which will be described in detail in 

 a forthcoming paper. The coils of the galvanometer in series 

 measured 140 ohms resistance. When the period of the 

 needb was reduced for the single swing to 10 seconds, one 

 millimetre deflexion on the scale indicated a current of 

 1*3 x 10 -11 ampere. With this degree of astaticism the zero- 

 point of the needle was perfectly constant. 



The bolometer with which the following determinations 

 were made is described as No. 2 in the previous paper. It 

 consisted of two strips of platinum, 12 millim. long and 

 0'05 millim. wide, each having about 80 ohms resistance, 

 only one of them being exposed to radiation. With the aid 

 of the new galvanometer we were able to trace a sensitiveness 

 of 0*000003° C. per millimetre deflexion. A standard candle 

 one metre distant produced a deflexion of 400 millim. 



With the exception of the changes here mentioned all 

 pieces of apparatus were identical with those formerly de- 

 scribed. The relative positions, moreover, of the instrument, 

 as well as the manner and the order in which the operations 

 were performed, v\ere retained unchanged. We can therefore 

 pass at once to the results of our observations. 



Measurements were made upon the three materials so well 

 known for their diathermanous properties — rock-salt, sylvite, 

 and fluorite. 



