OPTICAL IMAGES. 
Take any magnifying glass (the object lens unscrewed from an 
opera glass, or the spectacle glass, or eye-glass of a weak-sighted 
person will answer the purpose), and holding it with its surfaces 
vertical, let the flame of a candle be placed near it in its axis, and 
let a white card be held behind it at right angles to its axis. Let 
the card be moved gradually from the glass until the inverted 
image of the flame of the candle is seen distinctly upon it. In 
this position the flame may be supposed to be the object o'", and 
its image on the card the image i'". Let the candle be now re¬ 
moved a little farther from the glass. The image will become 
indistinct, but if the card be removed a little towards the glass, 
its distinctness will be restored. The flame will now represent 
o”, and its image on the cardi". See fig. 22, p. 97. 
In the same manner, if the candle be continually removed from 
the glass, its image will approach continually to the glass, but at 
a slower and slower rate. When, however, the flame has been 
withdrawn to the distance of several yards from the magnifying 
glass, its image will become sensibly stationary, never approaching 
in any perceptible degree closer to the glass, however far the candle 
may be removed. 
32. It must be observed, nevertheless, that although the posi¬ 
tion of the image of the flame remains thus unchanged by the in¬ 
creased distance of the candle from the glass, its magnitude under¬ 
goes a very perceptible change, decreasing in linear dimensions in 
exactly the same proportion as the distance of the candle from the 
lens increases. 
It appears, then, in fine, that when a convex lens is*presented 
to any object, whose distance from it exceeds a certain limit, the 
optical image of such object will be formed at a fixed distance behind 
the lens, which distance will be the same whatever the distance of 
the object may be. Thus, for example, if the lens be presented to a 
window looking out over a landscape, the image of this landscape 
will be seen depicted, but inverted in position on a card held 
behind the lens, at the fixed distance from it, which has just 
been indicated ; and although the trees, buildings, and mountains, 
which form the view before the lens, are at extremely various 
distances, their images will be all depicted on the card upon a 
small scale, at precisely the same distance from the lens. 
33. The point in the axis of a lens, at which a distinct picture 
of distant objects is thus produced, is called the pbincipal 
bogus * of the lens, and the distance of this point measured upon 
the axis from the lens is called the bocal length of the lens. 
* In some practical works on the microscope, this point is called the 
sidereal or solar focus. This term has not, however, obtained a place 
in the nomenclature of scientific writers. 
$8 - : . . 
