L. Bell — Absolute Wave-length of Light. 349 



mentioned were used on the large spectrometer in which the 

 telescopes were kept at a fixed angle and the grating was turned. 

 This method is, of course, applicable only to very solid instru- 

 ments in which the angle can readily be kept constant, and it 

 should be further noted that it also requires the use of very 

 perfect gratings, since the grating is used asymmetrically. As 

 a result of this the spectra on the two sides differ in dispersion, 

 and if the ruling is irregular either in spacing or in contour of 

 the individual lines, may differ quite widely in focal length, 

 definition and illumination. After critical examination gra- 

 tings III and IT appeared to be so nearly perfect in ruling, as 

 to be quite secure from the dangers of the method. The method 

 has moreover the distinct advantage of enabling the angle of 

 deviation to be varied within certain narrow limits. Hence it 

 becomes possible so to arrange the apparatus as to give to some 

 convenient line a double reflection that shall be an exact sub- 

 multiple of 360°. This once accomplished it becomes an easy 

 matter completely to eliminate the errors of the divided circle 

 and obtain a value of n<p, dependent only on the micrometer 

 constants, which in turn may be themselves almost eliminated. 

 To be sure, this method practically confined observations to the 

 spectra of a given order and limits the choice of lines for meas- 

 urement, but the first objection does not apply to gratings of 

 which the ruling is very nearly perfect, and since the relative 

 wave-lengths of a large number of lines are known with very 

 great exactness, measurements of the absolute wave length are 

 quite comparable even if made on different lines. 



As regards the constancy of the angle between the collimator 

 and observing telescope there was every reason to expect entire 

 permanence throughout the experiments, and observation soon 

 justified this expectation. The telescopes were firmly secured 

 at both ends to one and the same casting, which in turn was 

 firmly bedded in a brick pier. In addition the size of the appa- 

 ratus was such that a variation of even \" in the angle was quite 

 improbable. The angle measured in the ordinary way with a 

 collimating eye-piece could be determined to 1" of arc, exclu- 

 sive of errors of graduation in the circle. At first there ap- 

 peared to be distinct variations in the angle as determined at 

 the beginning of each series of observations, reaching some- 

 times more than 10". It soon appeared however that when the 

 same part of the circle was used the angle between the tele- 

 scope was sensibly the same and the apparent variations were 

 then traced to a periodic error in the divided circle, which by 

 the method of repetition was completely eliminated from the 

 measurements of angles of deviation and only appeared in the 

 determinations of d. This error was finally eliminated by meas- 

 uring d in various portions of the circle. 



