160 Prof. S. P. Langley on 



salt-lenses of 75 millim. aperture and 350millim. focus ; this 

 small ratio of aperture to focus in the lenses being required 

 to economise the feeble heat as much as possible. The prism 

 used with them is first set to minimum deviation on "some 

 visible line, and then automatically kept there for the invisible 

 ray under consideration. 



We owe these specimens of rock-salt to the particular kind- 

 ness of Prof. Hastings, of Yale College, and their extremely 

 exact surfaces to Mr. Brashear, of Allegheny, the maker of 

 the surfaces on which the Rowland gratings are ruled. On 

 this portion of the apparatus alone very great labour has been 

 expended. 



We have procured, through Mr. Brashear's skill, by means 

 previously described, a rock-salt prism having a field filled 

 with Fraunhofer lines, and showing distinctly the nickel line 

 between the D's. This is when first polished as it comes 

 from the maker's hands, but owing to the deliquescent nature 

 of the material, with the utmost care, the surfaces rapidly 

 deteriorate. As it is necessary for the precision of these re- 

 searches to determine the refracting angle of the prism, and 

 also the indices of refraction of some of the principal lines in 

 its visible and invisible spectrum, with a high degree of 

 accuracy, and as all these labours have to be repeated when 

 the prism is repolished, some idea of the labour in this portion 

 alone may be understood, when it is stated that the prism 

 has been sent to the maker and entirely refigured, and its 

 principal constants redetermined by us no less than thirteen 

 times in the past fourteen months, or since the 1st of January 

 1885*. 



We now give two examples of actual measurement of wave- 

 lengths ; the first, that in sunlight in a flint prism, which we 

 have designated as No. 2 ; the second in the rock-salt prism 

 just mentioned. 



First example of Measurement. With Flint-glass prism. 

 (Extract from original record.) 



Station, Allegheny. 

 Date, March 3rd, 1886. 

 Temperature of apparatus = 1°* 8 Cent. 

 State of sky, clear overhead, cirrus clouds near the horizon. 

 Aperture of slit Si = 2 millim. 

 „ „ S 2 = l millim. 



Lenses of glass (non achromatic), focal length = 800 millim. 

 for visible rays. 



* For a full description of the constants of this prism see this Journal 

 for May 1886. 



