42 
OPTICAL EFFECTS. 
by the particles of air placed between the mountains and tho 
eye of the observer. As wo withdraw from the isle of Tenc- 
riffe, the Piton or Sugar-loaf is seen for a considerable space 
of time in a positive manner, because it reflects a whitish 
light, and clearly detaches itself from the sky. But as this 
cone is only SO toises high, by 40 in breadth at its summit, 
it has recently been a question whether, from the diminutivc- 
uess of its mass, it can he visible at distances which exceed 
10 leagues; and whether it be not probable, that navigators 
distinguish the peaks as a small cloud above the horizon, only 
when the base of the Piton begins to be visible on it. If we 
admit, that the mean breadth of the Sugar-loaf is 100 toises, 
we find that the little cone, at 40 leagues distance, still 
subtends, in the horizontal direction, an angle of more than 
three minutes. This angle is considerable enough to render 
an object visible ; and if the height of the Piton greatly ex- 
ceeded its base, the angle in the horizontal direction might 
bo still smaller, and the object still continue to make an 
impression on our visual organs ; for micrometrical observa- 
tions have proved that the limit of vision is but a minute 
only, when the dimensions of the objects are the same in 
every direction. We distinguish at a distance, by the eye 
only, trunks of trees insulated in a vast plain, though the 
subtended angle be under twenty-five seconds. 
As the visibility of an object detaching itself in a brown 
colour, depends on the quantities of light which the eye 
meets on two lines, one of which ends at the mountain, and 
the other extends to the surface of the aerial ocean, it follows 
that the farther we remove from the object, the smaller the 
difference becomes between the light of tho surrounding 
atmosphere, and that of the strata of air before tbo mountain. 
For this reason, when less elevated summits begin to appear 
above the horizon, they present themselves at first under a 
darker hue than those wo discern at very great distances. In 
the same manner, the visibility of mountains seen only in a 
negative manner, does not depend solely on the state of the 
lower regions of the air, to which our meteorological obser- 
vations are limited, but also on the transparency and physical 
constitution of the air in tho most elevated parts ; for the 
image detaches itself better in proportion as the aerial light, 
which comes from flic limits of the atmosphere, has been 
