392 PROCEEDINGS OF THE AMERICAN ACADEMY. 



very closely, up to within a distance of the band about equal to the dis- 

 tance of the D3 line of helium. If we attempt to go closer than this we 

 immediately find discrepancies, which become larger as we approach the 

 D lines. 



The constant m in the above formula can be determined from a single 

 observation of the refractivity. It was determined from two values, 

 namely, the refractivity for the green line of mercury (A = 546) and 

 that for X = 585, the latter wave-length being quite close to the D lines. 

 Since the refractivity has widely different values for these two wave- 

 lengths, we should expect the difference between the two calculated val- 

 ues to be a maximum in this case, in the event of the dispersion being 

 incorrectly represented by the formula. The values for m found in the 

 two cases were 0.000056 and 0.000054, a surprisingly close agreement. 

 The mean value m = 0.00055 was taken, and the indices for a number 

 of wave-lengths calculated. Some of these values are given in the table 

 of refractive indices, and they will be found to agree very closely with 

 the observed values. The values calculated between the helium line and 

 the D lines are given in the second part of the table. 



It will be noticed that the greatest discrepancies are found in the ultra- 

 violet, the observed values indicating a greater deviation (negative, since 

 the index is less than unity) than we should expect theoretically. In 

 other words, we should expect the deviated spectrum to be more nearly 

 in coincidence with the undeviated than was found in the photographs 

 taken through the slotted slide. I am inclined to attribute the discrep- 

 ancy to the presence of hydrogen in the tube. The dispersion tube was 

 exhausted and sealed off from the pump before introduction into the 

 combustion furnace. A small amount of hydrogen is always liberated 

 from the sodium as soon as it is heated, and without doubt the tube con- 

 tained a small amount of this gas during the experiment. At the time it 

 was not expected that this would give any trouble, but observations ap- 

 pear to indicate the contrary. A tube containing air, or any ordinary 

 gas, acts like a prism with its thin edge down, when the floor of the tube is 

 strongly heated (compare with mirage). This is owing to the rarefaction 

 of the gas along the heated floor. Now the sodium prism is turned in ex- 

 actly the opposite way, the denser portions being close to the floor. But 

 on the blue side of the sodium absorption band the index of refraction is 

 less than unity, consetiuently the oppositely oriented prisms (hydrogen and 

 sodium vapor) deviate in the same direction, and the effect of the hydrogen 

 prism will be greatest in the ultra-violet for obvious reasons. The result 

 will be that slightly too great deviations will be found in this region, 



