164 Prof. R W. Wood. 



off the upper and lower portions, utilizing only the light which passes 

 in the median plane, where the change in density appears to be quite 

 uniform, this method having been used in preparing the dispersion 

 curve alluded to above. The effective angle lies probably somewhere 

 between 90 and 130, but even this estimate is mere guess work. 



If a single observation could be made with a vapour prism of known 

 angle, in which we could be sure that the density was uniform, 

 quantitative values could be assigned to determinations made with the 

 dispersion tubes. I have accordingly tried in every way possible to 

 obtain a prism fulfilling the required conditions. These endeavours 

 have thus far met with only partial success, but an account of the 

 devices tried may be of value to others working along similar lines. 



Glass and mica are so quickly attacked by the sodium vapour that 

 the use of these substances for prism faces seems to be out of the 

 question. Added to this there is the difficulty of making gas-tight 

 joints between plate-glass and metal which will stand a temperature 

 but little below a red heat. Repeated failures to secure prisms in 

 this way compelled me to seek some other method of giving the 

 vapour the required form. I had observed that in glass tubes held 

 before sodium flames the black vapour retreated before the approach 

 of a cold obstacle. This suggested to me that it might be possible to 

 do away with glass surfaces entirely, moulding the vapour into the 

 required prismatic form by the proximity of cold bodies. Experi- 

 ments along these lines were partially successful. Two pieces of thick 

 walled iron tubing, the ends of which had been cut off at an angle, 

 were introduced into a glass tube, and the sodium placed in the clear 

 space between the bevelled ends, as is shown in fig. 5. The ends of the 



FIG. 5. 



tube were closed in the usual manner and the whole mounted between 

 the collimator and telescope, in such a position that the prism formed 

 by the sloping ends of the iron tubes stood with its refracting edge 

 vertical. This was necessary, for in any other position the refraction 

 due to the non-homogeneity of the vapour would have made itself 

 felt. With a vertical slit and a vertical prism no lateral deviation 

 could result from this cause. It was hoped that the sodium vapour 

 formed by sudden and rapid heating of the glass tube would refuse 

 to enter the colder iron tube, and that a prismatic form would result 

 from the bevelled ends. The slit of the spectroscope was illuminated 

 with monochromatic light obtained by prismatic analysis, the wave- 



