OPTICS. 
the bason when empty, and walk back till 
you have just lost sight of the money, which 
will be hidden by the edge of the bason.Then 
pour water into the bason, and you will see 
the money distinctly, tliough you look at it 
-exactly from the same spot as before. See 
(fig. 2) where the piece of money at S will 
appear at L. Hence also the straight oar, 
when partly immersed in water, will appear 
bent, as A C S. 
If the rays of light fall upon a piece of flat 
glass, they are refracted into a direction 
nearer to the perpendicular, as described 
above, while they pass through the glass ; 
but after coming again into'air, they are re- 
fracted as much in the contrary direction ; 
so that tliey move exactly parallel to what 
they did before entering the glass. But, 
on account of the thinness of the glass, this 
deviation is generally overlooked, and it is 
considered as passing directly through the 
glass. 
If parallel rays, ab (fig. 1) fall upou a 
plano-convex lens, cd, they will be so re- 
fracted, as to unite in a point, c, behind it ; 
and this point is called the “ principal 
focus,” or the “ focus of parallel rays tlie 
distance of which from the middle of the 
glass, is called the “ focal distance,” which 
is equal to twice the radius of the sphere, of 
which the lens is a portion. 
When parallel rays, as A B (fig. 5) fall 
upon a double convex lens, they will be re- 
fracted, so as to meet in a focus, whose dis- 
tance is equal to the radius or semi- 
diameter of the sphere of the lens. 
Ex. 1. Let the rays of the sun pass 
through a convex lens into a dark room, and 
fall upon a sheet of white paper placed at 
the distance of the principal focus from the 
lens. 2. The rays of a candle in a room from 
which all external light is excluded, passing 
through a convex lens, will form an image 
on white paper. 
But if a lens be more convex on one side 
dian on the other, the rule for finding the 
focal distance is this : as the sum of the 
semi diameters of both convexities is to the 
semi-diameter of either, so is double the 
semi-diameter of the other to the distance of 
the focus ; or divide the double product of 
the radii by their sums, and the quotient 
will be the distance sought. 
Since all the rays of the sun which pass 
through a convex glass are collected toge- 
ther in its focus, tlie force of all their heat is 
collected into that part ; and is in proportion 
to the common heat of the sun, as the area 
of the glass is to the area of the focus. 
Hence we see the reason why a convex 
glass causes the sun’s rays to burn after 
passing through it. See Burning glass. 
All those rays cross the middle ray in the 
focus f, and then diverge from it to the 
contrary sides, in the same manner as they 
converged in coming to it. If another glass, 
F G, of the same convexity as D E, be 
placed ill (he rays at the same distance from 
the focus, it will refract them so, as that, 
after going out of it, they will be all pa- 
rallel, as 6 c ; and go on in the same manner 
as they came to the first glass D E, but on 
the contrary sides of the middle ray. The 
rays diverge from any radiant point, as 
from a principal focus ; therefore, if a can- 
dle be placed at f, in the focus of the con- 
vex glass F G, the diverging rays in the 
space Ff G, will be so refracted by the 
glass, that, after going out of it, they will be- 
come parallel, as shewn in the space c b. If 
the candle be placed nearer the glass than 
its focal distance, the rays will diverge, after 
passing through the glass, more or less, as 
the candle is more or less distant from the 
focus. 
If the candle be placed further from the 
glass than its focal distance, the rays will 
converge, after passing through the glass, 
and meet in a point, wliich will be more or 
less distant from the glass, as the candle is 
nearer to, or further from, its focus; and 
where the rays meet, they will form an 
inverted image of the flame of the candle ; 
which may be seen on a paper placed in the 
meeting of the rays. 
Hence, if any object, ABC (fig. 6), 
he placed beyond the focus, F, of the con- 
vex glass, d ef, some of the rays which 
flow from every point of the object, on the 
side next the glass, will fall upon it, and 
after passing through it, they will be con- 
verged into as many points on the opposite 
side of the glass, where the image of every 
point will be formed, and consequently the 
image of the whole object, wliich will be 
inverted. Tims the rays, Ad, A e, Af, 
flowing from the point A, will converge in 
the space, d a f, and by meeting at a, 
will there form the image of the point A. 
'fhe rays, B d, B c, B /, flowing from the 
point, B, will be united at b, by the refrac- 
tion of the glass, and will there form the 
image of the point, B. And the rays, C d, 
C e,Cf, flowing from the point, C, will be 
united at c, where tliey will form the image 
of the point, C. And so of all the inter- 
mediate poiuts between A and C. 
If the object, A B C, be brought nearer 
