1864. | Physics. 157 
Px! PHYSICS. 
Lieut, Heat, anp Execrriciry. 
Liaut.—The cause of the scintillations of stars has long been a 
puzzle, not only to children, but to philosophers. Many explanations 
have been given, but none are quite satisfactory. Mr. A. Claudet * 
has thrown some new light upon this subject, by an instrument which 
he has devised, called the chromatoscope. He attributes the beautiful 
sparkling, with changing colours, exhibited by certain stars on a clear 
night, to the evolution in different degrees of swiftness of the various 
coloured rays they emit. These rays are supposed to divide during 
their long and rapid course through space, and we see them following 
each other in quick succession, but so rapidly that, although we see 
distinctly the various colours, we cannot judge of the separate lengths 
of their duration. Mr. Claudet’s instrument consists of a reflecting 
telescope, part of which is caused to rotate eccentrically in such a 
manner, that instead of a point a ring-like image of the star is seen. 
The rapidity of rotation is adjusted so that each separate colour 
given by the star is drawn out into a large segment of the ring, and 
in that manner the light from the star can be analysed as in a spectro- 
scope. 
In observing the rays of sunlight through a powerful spectroscope 
many additional lines are visible when the sun is near the horizon. 
These are called telluric rays, as they have been shown to owe their 
existence to some components of the earth’s atmosphere. Father 
Secchi, the Roman Astronomer, considers that aqueous vapour in the 
atmosphere is the principal cause of these telluric rays, and this 
opinion has been generally adopted by physicists: but M. Volpicelli ¢ 
now describes experiments to prove that these rays are independent of 
the presence or absence of aqueous vapour in the atmosphere. In our 
opinion his experiments are scarcely conclusive ; for it is quite reason~ 
able to suppose that the passage of light through 100 miles of atmo- 
sphere might produce effects which could not be imitated in a labora- 
tory experiment. 
The determination of the refracting power of various transparent 
liquids and solids, a matter of considerable practical importance, is 
usually effected by reference to certain well-known lines in the solar 
spectrum. It would be much easier to have recourse to the bright 
spectral lines of coloured flames, which are obtainable with ease at any 
time, whereas the employment of Fraiinhofer’s lines is dependent on 
the weather. For accurate experiments it is necessary to know the 
length of the waves for the differently coloured rays, and this informa- 
tion has been supplied by Dr. J. Miiller,t by means of one of Nobert’s 
well-divided glass screens. His results show that the length of wave 
* Phil. Mag.’ No. 175. 
+ ‘Cosmos,’ vol. xxiii. p. 430. 
t Poggendort’s ‘ Annalen,’ vol, exviii. p. 641. 
