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MAY BE SEEN AT SEA. 37 
ference of intensity that subsists between the light 
which surrounds them, and that reflected by the par- 
ticles of air placed between the object of vision and 
the observer. In receding from Teneriffe, the Sugar- 
loaf is long seen in a positive manner, as it reflects 
a whitish light, and detaches itself clearly from the 
sky; but as this terminal cone is only 512 feet 
high, by 256 in breadth at its summit, it has been 
questioned whether it can be visible beyond the dis- 
tance of 138 miles. If it be admitted that the mean 
breadth of the Sugar-loaf is 6394 feet, it will 
still subtend, at the distance now named, an angle 
of more than three minutes, which is enough to 
render it visible; and were the height of the cone 
greatly to exceed its basis, the angle might be still 
less, and the mass yet make an impression on our 
organs; for it has been proved by micrometrical 
observations, that the limit of vision is one minute 
only when the dimensions of objects are the same 
in all directions. 
As the visibility of an object, which detaches it- 
self from the sky of a brown colour, depends on 
the quantities of light the eye meets in two lines, 
of which one ends at the mountain and the other 
is prolonged to the surface of the aerial ocean, it 
follows that the farther we remove from the object, 
the less also becomes the difference between the 
light of the surrounding atmosphere and that of the 
strata of air placed before the mountain. For this 
reason, when summits of low elevation begin to ap- 
pear above the horizon, they are of a darker tint 
than those more elevated ones which we discover at 
very great distances. In like manner, the visibility 
of mountains which are only negatively perceived, 
