220 Prof. Dufour: Instructions for the better observation of 
proach the zenith; and reasoning from this fact, it may seem 
impossible to compare observations unless they have all been 
taken from stars at the same height. On comparing a vast 
number of observations taken under most favourable conditions, 
and when there had been no apparent atmospheric perturbations 
either on the preceding or following days, I have placed it beyond 
doubt that the scintillation does really decrease when the stars 
approach the zenith, and that, for any given height, the scin- 
tillation is sensibly proportional to the product obtained by mul- 
tiplying the depth of the stratum of air which the rays of light 
traverse, by the astronomical refraction for the height under con- 
sideration. 
Let us designate this product by P. Representing by 1 
the height of the atmosphere, and estimating the refractions in 
seconds, it will be found that for the different heights the results 
P are— 
Height of the star. Value P. 
° 
SO cm din ited txt” val wha ae 
BE Waves cule. alas versal Oe 
BOs tee gsinlsivwvides ot LOS 
Bo! OF leah od sone Tea 
MO phen Pahy seta eee aloe 
BSL SS eo: ao {Ce 
BOC HEC Coie Ro ieee 
FON tes ee he Ree 
GO iP RA ep eoatls +e eee 
GiiA SSRs ee ee 
Wut aCe die pia th chbis ieee 
FES Oo nat Pay Nee 
SOR: fF aii rhe eee alee 
85 wiste Bie 5-1 
These figures represent tolerably well the normal state of the 
scintillation at Morges, when the height of the star above the 
horizon varies from 20° to 75°. Below 20° the calculated value 
no longer corresponds with the observations, but the neighbour- 
hood of the horizon sufficiently explains this deviation ; and as 
to the stars situated at a height which exceeds 75°, their scintil- 
lation is in general so feeble that the slightest error in the ob- 
servation will cause a notable modification in the correspondence 
of the figures. Thus, if one has observed at a height of 60° a 
scintillation of 1°6, and one wishes to know what would have 
been, under the same circumstances, the scintillation of the same 
star if it had been only 45° above the horizon, one would obtain 
1:6 x 81°8 
eit bi. i 
Scintillation at 45°= 38-7 =3°4. 
