130 PHYSICS. 
forwards by a rack and pinion: p and q’ are square plates of brass, united at 
the corners by small posts. About each post is wound aspiral spring which 
presses a third plate, g, against qg’. The glass slide to which the object is 
attached is held between the two plates, g andq’. The lenses just described 
serve only to throw an intense light on the object. The magnifying is 
produced by means of a third lens, J, which should be achromatic. This 
also can be adjusted, with respect to the object, by a rack and pinion. 
Rays of light, passing from the highly illuminated object through the lens, ¢, 
will be refracted by it into an image, whose size depends on the distance 
from the lens at which it is received on a screen. Should this distance be 
ten or twenty feet, the image will be of enormous size. When, instead of 
solar light, that from the oxy-hydrogen blow-pipe is employed, we have the 
oxy-hydrogen microscope. 
The Magic Lantern (figs. 102 and 108) is the same in principle with the 
solar microscope. This consists of two lenses, behind which is placed a 
lamp, and between them the object. The light from the Jamp is 
concentrated by the first lens, and thrown on the object. This being in 
the focus of the second lens, has an image formed of itself on the other 
side, and falling on a wall or on ascreen placed at the distance of some 
feet. Here, as in the case of the solar microscope, the size of the image, 
the necessary adjustments being all made, will be greater with the distance 
of the screen from the Jens. What is gained in the size of the image, will. 
however, be lost in its brightness, this increasing with the proximity of the 
screen. A proper medium must therefore be selected which shall combine 
both elements of the picture. The construction of the accessory parts 
of the magic lantern will be sufficiently evident from the figure. 
Telescopes are divided into two classes, refracting and reflecting. In the 
former, the images of distant objects are formed by a convex lens; in the 
latter, by a concave mirror or speculum. The refracting telescope, again, 
may have different constructions. In the oldest of these, or the Galilean 
( fig. 65), the ocular is a single concave lens, XZ. VW is the objective, 
which, of itself, would produce an inverted and reduced image of a distant 
object at ab; the rays, however, before uniting in an image, fall upon the 
concave eye-glass, XZ. The eye-glass is so placed that the distance 
of ab is slightly greater than the dispersive distance of XZ; accordingly, 
rays converging to one point of ab, are refracted by the concave lens, so 
that after their passage through it, they diverge as if proceeding from a 
point before the glass. The rays converging towards b, diverge there as if 
coming from B; those converging from a, as if they came from A. An 
erect and magnified image, AB, is thus produced. The magnifying power 
of the Galilean telescope is as the ratio of the focal distance of the object- 
glass to that of the eye-glass. 
In the Astronomical or Keplerian Telescope ( fig. 66), the ocular, instead of 
being concave, is a convex lens. Here the objective produces an inverted 
image, ab, which is magnified by the ocular, in which case its image, still 
inverted however, is referred by the eye to AB. In the terrestrial telescope 
the ocular or eye-piece consists of four convex lenses, for the purpose of 
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