70 
CURIOUS Illusion. 
the illusion soon ceased, and we found that the luminous 
points were the images ot several stars magnified by the 
vapours* These images remained motionless at intervals, 
they then seemed to rise perpendicularly, descended side- 
ways, and returned to the point whence they had departed. 
This motion lasted one or two seconds. Though we had no 
exact means of measuring the extent of the lateral shifting, 
we did not the less distinctly observe the path of the lumi- 
nous point. It did not appear double from an effect ot mirage, 
and left no trace of light behind. Bringing, with the tele- 
scope of a small sextant by Troughtou, the stars into contact 
with the lofty summit of a mountain in Lancerota, 1 
observed that the oscillation was constantly directed to- 
wards the same point, that is to say, towards that part of 
the horizon where the disk ot the sun was to appear ; and 
that, making allowance for the motion of the star in its 
declination, the image returned always to the same place. 
These appearances of lateral refraction ceased long before 
daylight rendered the stars quite invisible. I have faith- 
fully related what we saw during the twilight, without un- 
dertaking to explain this extraordinary phenomenon, _ of 
which I published an account in Baron Zaeh’s Astronomical 
Journal, twelve years ago. The motion of the vesicular 
vapours’ caused by the rising of the sun; the mingling of 
several layers of air, the temperature and density of which 
were very different, no doubt contributed, to produce _ an 
apparent movement of the stars in the horizontal direction. 
We see something similar in the strong undulations of the 
solar disk, when it cuts the horizon ; but these undulations 
seldom exceed twenty seconds, while the lateral motion 
of the stars, observed at the peak, at more than 1800 
toiscs, was easily distinguished by the naked eye, and seemed 
to exceed all that we have thought it possible to consider 
hitlierto as the effect of the refraction of the light of the 
stars. On the top of the Andes, at Antiaana, 1 observed 
the sun-rise, and passed the whole night at the height oi‘21.00 
toises, without noting any appearance resembling this phe- 
nomenon. , , 
I was anxious to make an exact observation of the instant 
of sun-rising at an elevation so considerable as that we had 
reached on the peak of Teneriffe. jMo traveller, furnished 
