of Infra- Red Rays in Rock Salt and Sylvine. 43 



be present in the infra-red part of the spectrum, works for the 

 most part, continually on the radiometer, and therefore 

 becomes harmless for the observations. 



Because of the small dispersion of our prisms, we could, in 

 our experiments, only determine the general trend of the 

 absorption. In the field between \ = 9*0> and \ = 20*75/*, 

 measurements were taken at twelve different points, on the 

 absorption of a rock salt layer 23*3 mm thick and a sylvine layer 

 10*7 mm thick by placing alternately a thick and a thin plate 

 of the material to be examined, in front of the slit s . The 

 ratio of the throws of the radiometer, observed in both cases, 

 gives the transmission for a layer, the thickness of which is 

 equal to the difference in thickness of the two plates.* 



In this way the following results were obtained : 







Table III. 







Wave length. 



Rock salt 



Sylvine. 



Wave length. 



Rock salt. 



Sylvine. 



A 



d = 23-3 mm 



d = 10-7 mm 



A 



d = 23-3 mm 



d= 10-7 mm 



dju 



98-8 



100-2 



16yU 



38*1 



93-1 



10 



987 



98"6 



17 



21-4 



91-6 



n 



98-9 



989 



18 



4-94 



85-3 



12 



98-3 



99-4 



19 



O'l 



74-3 



13 



944 



99-4 



20-7 







56'3 



14 



84-7 



97-3 



23-7 







13-5 



15 



67-8 



95-0 









In another table we give our results on the transmission of a 

 rock salt plate, a chloride of silver platef and a fluorite plate, 

 which were obtained by comparison of the direct and trans- 

 mitted energies. In order to calculate the absorption of the 

 plates, the influence of the reflection must be eliminated from 

 these numbers. 



"With the help of the figures given in Table III, the follow- 

 ing values (Table V) were obtained for the transmission of 

 layers of rock salt and sylvine l cm thick. For rock salt, from 

 X = lVfji on. Table V was filled in with the results from Table 

 IV. Similarly, the values given for fluorite were calculated 

 from the values given in Table IV", after using Paschen's 

 measurements on dispersion in eliminating from them the loss 

 of energy due to reflection. 



* Since the absorption in rock salt was very considerable beyond 19// we used 

 only the syivine prism for the wave lengths A = 20, Ifi and A = 23'7/U, making the 

 rays monochromatic before their entrance into the slit <s , by successive reflection 

 on quartz (A = 2(r7//) and on fluonte (A = 23'7//). 



f The chloride of silver plate used by us is one of those which Schultze-Sellack 

 prepared and tested for transmission of heat rays in L870. — Pogg. Ann., exxxix, 

 p. 182, 1870. 



