110 Prof. Wood on the Dispersion^ Absorption, 



absorption-bands are present. If it had turned out that 

 m equalled unity, the inference would have been that the 

 absorption-band 'at X=43 was the only one present. The 

 large value of (v/i— 1) indicates that there are bands of 

 metallic absorption in the ultra-violet. As I have already 

 shown, a strong absorption-band exists a little below X = 20, 

 and a weak one at X = 25. 



Though the formula as it stands represents fairly well the 

 dispersion in the red, yellow, and green, it breaks down if we 

 try to apply it to the values found in the ultra-violet by the 

 photographic application of the method of crossed prisms. 

 This is due to the fact that we are getting into the region in 



which —2 — ^//2 ^^ ^^ longer approximately equal to unity. 



This quantity increases in magnitude as \ decreases, and 

 the values of n will consequently be higher than those calcu- 

 lated on the assumption that the quantity is equal to unity. 

 To meet this contingency we must use the formula (neglecting 

 the weak band at X = 25) 



^ = 1 + 7T^ ^-TcTx + 



Since ??z = 2*13 = w^^ + l, we can take m'' = l'13. 



The A'alue of the refractive index n for X=:34, calculated 

 from the original or simplified formula, is ?z = l-l, while 

 the observed value is 1*3 ; in other words, the value is 

 raised by the influence of the remote ultra-A'iolet band. 

 Using this observed value of ^, it is possible to calculate the 

 wave-lenoth V of the ultra-violet band, assumiiio-. as above. 



This was found to be X'^ = 18, a value which looks very 

 reasonable, judging from the photographs of the absorption 

 in this region. It is of course impossible to determine 

 experimentally the centre of this band, since the nitroso cuts 

 off everything below X = 20. Whether or not a return of 

 transparency would be found further along by employing 

 fluorite plates and a vacuum spectrograph, it is impossible 

 to say. 



We are now in a position to calculate other values of n in 

 the ultra-violet and compare them with the observed values. 



ForX=36 (the wave-length for which the lowest value 

 of the refractive index was found experimentally), n calcu- 

 lated by the first formula is n = 0"92 ; by second formula, 

 ?z = l*08 ; observed, ?2 = 1'05. For X = 31j by second formula 

 n = l'4:2 ; observed, w = 1-43. 



Obviously we cannot apply the formula to that portion of 



