L.Bell — Absolute Wave-length of Light. 267 



Fraunhofer' s second and more complete paper appeared in 

 1823 in Gilbert's Annalen (lxxiv, 337). Its title is : " Kurzer 

 Bericht von den Resultaten neuerer Versuche iiber die Gesetze 

 des Lichtes, nnd die Theorie derselben." This paper gives a 

 detailed account of his experiments with two glass gratings. 

 Of these, the grating spaces were respectively 0'0033 and 

 0'0160 mm . The former was apparently much the better, and 

 upon it Fraunhofer based his final result, which for D was 

 0*0005886 mm while the experiments with the coarser grating 

 gave - 0005890 mm . These values apply quite certainly to the 

 mean of the two D lines, and not, as has been sometimes 

 supposed, to one of them alone. 



The experimental work with these glass gratings was much 

 better than with the previous wire ones, since the angular de- 

 flections were very much larger and the gratings themselves 

 were susceptible of far more exact measurement. But at best 

 they were but indifferent instruments and the terminal lines 

 were so bad that they had to ' be retraced before the grating 

 space could be determined. So, between poor gratings and in- 

 different standards of length, Fraunhofer' s determination of 

 absolute wave length left very much to be desired. However, 

 nothing much better could be accomplished until the art of 

 making gratings was very much improved, and it was not until 

 ]STobert's gratings became tolerably well known, that any seri- 

 ous attempt was made to improve on Fraunhofer's results. 

 From time to time various investigators worked on the prob- 

 lem, both with Robert's earlier gratings and by utilizing vari- 

 ous interference phenomena. When, however, the great in- 

 vestigations of Bunsen and Kirchhoff revolutionized spectro- 

 scopic work and emphasized its great importance, the attention 

 of scientific men was called to the need for accurate measure- 

 ments, and for half a dozen years investigators were active, and 

 Mascart, Ditscheiner and Angstrom appeared on the field 

 almost simultaneously. Each published a paper in 1864, and 

 of these that of Mascart is probably the most accurate and 

 painstaking, though now it is quite certain that the values he 

 obtained were considerably too small. He employed four or 

 five of Robert's gratings and instead of placing the grating 

 perpendicular to either the collimator or the observing tele- 

 scope, used it in the position of minimum deviation, that is to 

 say, so that the plane of the grating should bisect the angle 

 formed by the incident and diffracted rays. This position has 

 certain advantages, but as the experimentation is rather more 

 difficult than in the ordinary position, the method appears to be 

 of somewhat questionable utility. It avoids, to be sure, 'the 

 necessity of placing the grating normal to the axis of either 

 telescope, but as there is very little trouble in making this ad- 



