S. P. Langley — Optical Properties of Rock-salt. 477 



While the identification is not complete the number of the 

 approximate coincidences renders it extremely probable that 

 the lines observed are those of the solar atmosphere. 



To quote the pioneer of American spectroscopic observa- 

 tions, it would seem that there are many stars in the same con- 

 dition as the sun, but with the corona more pronounced. 



Tale College Observatory, November, 1885. 



Art. LIX. — Note on the Optical Properties of Rock-salt; 

 by S. P. Langley. 



Since the first experiments of Melloni the optical proper- 

 ties of rock-salt have received comparatively little attention, 

 although this substance is every day coming more into use, as 

 the importance of the study of radiant heat is recognized. It 

 was asserted by Melloni that rock-salt is almost perfectly dia- 

 thermanous to all kinds of heat radiations, and -that it trans- 

 mits a little over 92 per cent, of the incident heat of whatever 

 kind. This statement was disputed by Provostaye and Desains,* 

 who maintained that there was considerable difference in its 

 absorptive action toward heat radiated from sources differing 

 widely in character ; still, even admitting the claims of these 

 physicists, the great difference between the action of this sub- 

 stance and others, such as glass, is very striking, and we are 

 dependent chiefly on it for our analysis of the action of obscure 

 heat. 



In certain researches which have been made of late at the 

 Allegheny Observatory on the distribution of heat in the spec- 

 trum of the moon, and of terrestrial sources of radiation at 

 very low temperatures, the use of an exceptionally perfect rock- 

 salt train has been sought in order that heat deviations meas- 

 ured with a precision comparable with that of optical work 

 might be secured, at the same time that the extremely feeble 

 radiations at command should suffer the least possible amount 

 of absorption in the apparatus. After long searching, blocks of 

 rock-salt were finally obtained through the great kindness of 

 Professor C.. S. Hastings, of Yale College, from which two 60° 

 prisms were cut of about 64 millimeters on a side, and lenses 

 of nearly 75 mm aperture. The most perfect prisms the writer 

 could obtain in Europe, did not distinctly show a single Fraun- 

 hofer line, and he was assured by opticians there that no rock- 

 salt prism ever did or could do more. He is happy to say that 

 the skill of our American opticians has produced what was pro- 

 nounced impossible, — a rock-salt prism which shows the Fraun- 

 hofer lines with all the sharpness of flint glass. Such prisms 

 * Coinptes Rendus de l'Academie des Sciences, xxxvi, p. 84. 



