40 H. Rubens and B. W. Snow on the Refraction of 



the point of intersection in question lies in such a way that 

 the adjacent minim-urn (mth order) lies on the side of the 

 longer wave-lengths, and the adjacent maximum on the side 

 of the shorter wave-lengths, then ?i = 2m. 



The introduction of these points in the calculation of the 

 curve of dispersion made it possible for us to conduct the 

 observations with interference-bands as broad as were necessary, 

 and at the same time to obtain a sufficiently great number of 

 points to enable us to ascertain the character of the curve of 

 dispersion with nearly the same degree of accuracy as in those 

 portions of the spectrum lying but little beyond the reach of 

 the eye. 



At this point mention should be made of a peculiarity of 

 the energy-curve, which may be observed in the drawing 

 (fig. 1). The deflexions of the galvanometer, at the point of 

 the last minimum a 11? not only sink to zero, but even assume 

 negative values. The cause of this singularity, which also 

 appears to a smaller degree in the energy-curve of fluorite, is 

 to be found in the fact that the second unilluminated arm of 

 the bolometer, which was placed in the apparatus within a 

 casing of hard-rubber, notwithstanding this covering received 

 upon its surface a greater amount of energy than the first 

 arm, which was exposed to the direct radiation. The plausi- 

 bility of this explanation is increased when we remember that 

 the covered resistance is then at a portion of the spectrum 

 in which the mean energy is 50 times greater than in the 

 neighbourhood of the minimum a lh and that ebonite is not 

 opaque to thermal radiation of great wave-length. 



In the following Table are found the results of the observed 

 indices of refraction and wave-lengths. The first column, 

 entitled " Name,''' gives the quality of the characteristic point 

 in question, as Fraunhofer line, minimum (a), maximum (b), 

 or point of intersection (c) of the curves G and E, ; the second 

 column contains the angle of deviation a as measured on the 

 graduated circle ; the third contains the index of refraction n, 

 calculated from the refracting angle <p and the angle of 

 deviation a according to the formula 



. + « 



sin 



n = 



sm| 



the fourth column contains finally the wave-length which is 

 calculated from the order m of the interference -band and the 

 constant K = 2dcos?. The curve of dispersion plotted from 

 the data of this Table is found in fig, 4, a. 



