382 PROCEEDINGS OF THE AMERICAN ACADEMY. 



raeut of the apparatus had occurred. A cadmium spark aud sodium flame 

 were now placed before the slit of the grating spectroscope, and the 

 position of the bright lines recorded on the plate. The deviations which 

 different regions of the spectrum had experienced were then carefully 

 measured. The object of making a number of records was to show 

 whether the relative deviations depended on the temperature of the tube 

 and density of the vapor. No indications of any such irregularity were 

 found. The values found in this way agreed admirably with the results 

 obtained with the interferometer. Tt moreover furnished data regarding 

 the dispersion in the blue and violet region, which the interferometer failed 

 to do with any accuracy for reasons which I have given. Photographs 

 of the dispersed spectrum are reproduced in Figure 8 (Plate III). 



Numerical Results. 



The results given by the three methods are given in the second column 

 of the table in the section devoted to the refraction and dispersion of 

 sodium vapor of great density, which follows. For the sake of uni- 

 formity they have all been reduced to the same scale. A shift of 100 

 helium (Dg) fringes being taken as the standard. A wave-length, for 

 which the relative shift, with respect to helium light, of the interferom- 

 eter fringes was found to be 1 : 4, is entered in the table as 25. 



The shift ratio for the green line of mercury witli respect to helium 

 was found to be 1 : 25. The wave-length of this light is therefore repre- 

 sented in the table by the numeral 4, which means that a prism of sodium 

 vapor which gives a deviation 4 for green hg. light, gives a deviation 

 100 for helium light. Tlie results obtained by the methods of crossed 

 prisms (both visual and photographic) were reduced to the same scale, 

 being compared with the value found for the green mercury line. This 

 was necessary, since if a vapor of sufficient density is used to give a meas- 

 urable deviation in the blue and violet region, it absorbs almost entirely 

 that portion of the spectrum occupied by the helium line. The values 

 obtained by means of the Zeeman doublet and the two yellow lines of 

 mercury, by the method of coincidences are not given in the table. 

 As their values were purely relative, i. e. not compared with helium or 

 any other standard wave-length, it was necessary to assign to one of the 

 wave-lengths a shift taken from the curve determined by the other 

 methods, and then calculate the shift of the other wave-length. For 

 example, in the case of the yellow mercury lines, of wave-lengths 577 and 

 579, the ratio as obtained by the coincidence method was 5 : 6. The 

 shift of 577 on the curve was found to be 14; i.e. 14 of these fringes 



