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AMERICAN JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



Art. IX. — On hitherto unrecognized Wave-lengths; by S. P. 

 Langley. (With Plates I to IY.) 



We have already* presented a description of the method by 

 which we were able to fix the wave-lengths in the solar 

 spectrum by direct measurement as far as about 23,000 of 

 Angstrom's scale. At this point the heat in the solar normal 

 spectrum had become so feeble that it taxed the utmost 

 limits of our capacity in 1881 to measure it ; for it will 

 be remembered that we are able to study the prismatic 

 spectrum of the infra-red with comparative ease, because 

 the prism condenses the heat; but the grating greatly 

 diffuses and weakens it, so that, were it due to this cause 

 alone, we should find measures in this part of the grating 

 spectrum enormously more difficult than those in the prismatic. 

 But independently of this, of the heat which belongs to any 

 ray, our grating in general employs not over the tenth part. 

 These causes combine to make the heat in certain portions, 

 where we hnve been compelled to measure, almost infini- 

 tesimal. 



We are led to take this labor, not primarily to settle the 

 theoretical questions involved in determining the relation be- 

 tween dispersion and wave-length, (though these are most inter- 

 esting), but with the object of providing a way which will here- 

 after enable any observer to determine the visible or invisible 

 wave-lengths of any heat, whether from a celestial or terrestrial 



*This Journal, vol. xxvii, March, 1884. 



A.M. Jour. Scl— Third Series, Vol. XXXII, No. 188.— August, 1886. 

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