128 PHYSICAL PHENOMENA OF UPPER REGIONS OF ATMOSPHERE. 



less refringent, and an upper one of diluted glycerin, lighter but also 

 less refringent than the middle one. A movable mirror, LL, throws a 

 beam of light through an opening, S, of a diaphragm. By throwing 

 this beam in appropriate directions, it is reflected either from the upper 

 or the lower stratum. A little fluorescein lights up the trajectories of 

 the beams and makes their curvatures quite visible; we can thus 

 represent the Alpengluhen with some accessory features. 



B 



Scintillation of the stars. — This phenomenon is also a proof of the 

 alterations of temperature and movement that occur in the higher 

 strata of air. Spectrum analysis shows that the scintillation is pro- 

 duced by the disappearance of the successive colors of the spectrum 

 following a somewhat regular course, according to the distance of the 

 star from the zenith. 



Imitation of this phenomenon. — A very striking experiment showing 

 this can be made by projecting with a lens, L, the image of a luminous 

 opeuiug, O, upon a small silvered ball, B, 3 to 4 inches in diameter, 

 placed upon black velvet. We thus obtain the appearance of a fixed 

 star of remarkable brilliancy. 



Fig. 3. 



But the luminous opening, O, is made in a cardboard upon which is 

 projected the spectral image of a slit, F, dispersed by a prism for 

 direct vision, P. The cardboard, CO, is not exactly at the focus of 

 the spectrum ; that being formed farther away, in the plane of the lens, 

 L. It hence results that the iridescent image of the slit in the card- 

 board has at its middle, where is placed the opening, O, a white region. 

 The light thrown upon the ball, B, is therefore perfectly colorless. But 

 on leaving the opening, the beam expands into a spectrum upon the 

 projection lens, L, which recomposes it at B, as in the celebrated 

 Newtonian experiment. 



Then by shifting before the lens, L, a grating with large meshes, 

 certain radiations are intercepted, and the star, B, appears colored. 



A diverging half lens, D, having the same focus as L, annuls its 

 effect, and the spectrum of the star, with the artificial bauds caused by 



