﻿404 Intelligence and Miscellaneous Articles. 



The author intends in due course to report upon comparative 

 measurements with mercury barometers and this instrument. — Pog- 

 gendorff's Annalen, February 1871. 



Ofen, December 28, 1870. 



INVERSION OF THE SODIUM-LINE. BY A. WEINHOLD. 



The usual mode of inverting the sodium-line (in which feebly lu- 

 minous sodium vapour is interposed between an ignited body and 

 the slit of the spectroscope) requires great brightness in the white 

 light, in order that the quantity of light absorbed by the sodium 

 vapour may be considerably greater than that radiated by it, and 

 thus the sodium-line be considerably feebler than the adjacent parts 

 of the spectrum. 



It was to be expected that the inversion would take place more 

 readily if the brightness of the parts of the spectrum adjacent to 

 the sodium-line could be as much increased as the brightness of the 

 sodium-line is increased by the luminosity of the sodium vapour. 



This can be very easily effected. A small petroleum lamp is 

 placed in front of the slit of a small spectroscope consisting merely 

 of a tube (without lens) and a powerfully dispersing prism ; and a 

 spirit-flame intensely coloured by common salt is so placed between 

 the spectrum and the eye that it covers the entire spectrum ; there at 

 once appears a very distinct dark sodium-line, while the same spirit- 

 flame placed between the petroleum lamp and the slit brings out 

 the sodium-line bright and luminous. 



In the first position the entire spectrum seems illuminated by the 

 yellow light indicated by it, and therefore the sodium-line is less bright 

 than the adjacent parts of the spectrum by the entire amount of the 

 absorption ; while in the second position these parts are not at all 

 altered pr the sodium-line enfeebled by the absorption of sodium 

 vapour, but strengthened by its own radiation. 



If the lamp- wick be only rubbed with salt, it is only in the first 

 few seconds after lighting that it is intense enough to produce a 

 distinct inversion ; it is better before filling the lamp with alcohol 

 to dilute this with water and then saturate it with salt ; occasional 

 rubbing of the wick between the finger is advantageous even here, to 

 make the sodium-line of a dense black. 



There is a slight difficulty which very short-sighted people expe- 

 rience in finding the right accommodation for the slit, and not for the 

 alcohol-flame. This difficulty is lessened by placing a wire in front of 

 this slit, which then appears as a dark horizontal line in a continuous 

 spectrum, and shows how the proper accommodation is to be effected. 



The use of a spectroscope with telescopes gave no good results. 

 Owing to the small distance of the eye, the flame cannot be placed 

 between the eyepiece and the eye ; and placed between the prism 

 and the object-glass it confuses the picture too much, owing to the 

 currents of hot air. From the construction of my apparatus an ad- 

 justment in an incision of the slit-tube close behind the slit, or be- 

 tween the object-glass and eyepiece of my telescope near the col- 

 lecting-lens, was not possible. — Poggendorff's Annalen, Feb. 1871. 



