Refractive Indices of Liquid Nitrogen and Air. 329 



gradually and generally giving off streams of small bubbles. 

 These bubbles do not sensibly mar the distinctness of an 

 object viewed through the liquid with the naked eye, but we 

 found that they did make a difference when a telescope was 

 used. As we had no double vessels with a vacuous interval 

 except such as were either globular or cylindrical, we had to 

 meet the difficulty that such vessels when filled with liquid 

 acted like lenses, and lenses which were irregular and full of 

 striations. In fact we could not by the use of compensating 

 lenses obtain in the field of a small telescope any tolerably 

 definite image of a candle, or other source of light, seen 

 through the vessel. At the same time, when the pair of 

 glass plates above mentioned was suspended in the liquid and 

 a sodium flame viewed with the unaided eye through them, 

 the extinction of the light, when the plates were turned to 

 the position for total reflexion, appeared quite sudden. The 

 use of a telescope, however, showed that the rays forming 

 the blurred image, even when limited by screens to the 

 central part of the globe, or cylinder, really passed in so 

 many different directions that they were by no means all 

 extinguished at once. 



In order to avoid the necessity of observing any image at 

 all through the liquid, we made use of it simply as a lens to 

 concentrate the light observed on the slit of a spectroscope. 

 The arrangement is shown in the annexed figure, where a is 

 the source of light, either a sodium-flame or an electric 

 spark, b a screen with a slit, c the outer glass vessel, d the 



inner glass vessel with the liquid, e the pair of glass plates 

 immersed in the liquid,/ the slit of a spectroscope, gg screens 

 of black paper on the outside of the outer vessel. 



The distances of a and / were arranged so that they should 

 be conjugate foci when the vessel was midway between them 

 The pair of glass plates were separated by a narrow ring of 

 thin filter-paper, thoroughly wetted with white of egg and 

 allowed to dry. They were held in a small brass clip 

 attached to a rod, which formed the prolongation of the 

 vertical axis of a theodolite, by which the angle through 

 which they were turned was measured. They were adjusted 



Phil. Mag. S. 5. Vol. 36. No. 221. Oct. 1893. Z 



