540 Prof. Trowbridge and Mr. Crandall on Groove-Form 



element, absent order?, &c, are considered, as will be seen 

 later *. 



Fig-. 3. 



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Energy from the Hernst Lamp 

 Reflected by a Gold Mirror. 



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* There is a further correction, due to the fact that with a setting of 

 the grating so that orders to one side of the central image are on the slit 

 of the vacuum spectrometer, the dispersions of grating and prism are 

 additive, while with orders on the other idde of the central image, the 

 dispersions oppose one another. Thus of two equal maxima, one on 

 each side of the central image, one would appear more spread out than 

 the other, if this effect were noticeable ; however, this correction was 

 worked out from the dimensions of the slit, dispersion of the grating 

 and rock-salt prisms, and found to be vanishingly small (-cT'OOl/*) in 

 comparison with either of the two above mentioned corrections, and 

 therefore negligible in this work. 



