OPTICS. 131 
exhibiting objects erect. To determine the magnifying power of the 
astronomical telescope, divide the focal distance of the object-glass by that 
of the eye-glass. As, however, this focal length may not always be known, 
the magnifying power may be determined by experiment. Set up at some 
distance from the telescope a graduated staff. Look at the staff with one 
eye unarmed, and with the other through the telescope, simultaneously. 
By observing how many degrees of the staff, as seen with the naked eye, 
correspond to one as seen magnified through the telescope, we shall be able 
to determine the magnifying power. 
The essential part of the reflecting telescope consists of a concave mirror 
or speculum. This, when placed before the object, produces an inverted 
image of it, which may be viewed in various ways. In the Gregorian 
telescope (fig. 62), the concave mirror, mm’, has a circular aperture, cc’, in 
the centre. Rays falling upon the mirror are reflected so as to produce an 
inverted image of the object at 72’, just in the focus of a second small 
concave mirror. This again inverts the image, casting it erect before the 
ocular. The ocular generally consists of two lenses,—an eye-glass proper, 
and a field glass for increasing the field of vision. By means of the screw 
bs, the position of the smaller mirror with respect to the ocular may be 
varied. . 
In the Cassegrainian telescope, instead of the small concave mirror a 
convex one is employed (fig. 63). This mirror receives the rays from the 
concave mirror before their union. In this way an inverted image, 21, of 
the object is formed between the two lenses of the ocular, to be further 
magnified by the eye-piece. 
In the Newtonian telescope (fig. 64), the rays reflected from the concave 
mirror fall on a plane mirror placed at an angle of forty-five degrees, and are 
cast into the axis of the ocular, placed in the side of the tube at 0. In this 
arrangement there need be no perforation of the large mirror. For further 
information respecting telescopes we would refer our readers to the article 
Astronomy. 
g. Of the Interference and Diffraction of Light. 
Two very different hypotheses have been suggested as to the actual 
nature of light. That of emission, or emanation, assumes the existence of 
an exceedingly rarefied matter, emitted or projected from a Juminous body 
in every direction. The hypothesis of oscillations or undulations, on the 
other hand, supposes an almost inconceivably subtle medium called ether, 
which fills all space, even the pores of bodies, oscillations in which produce 
the physical phenomena called light. At the present time the latter is most 
generally adopted, though the former dees not lack the authority of great 
names, among which that of Newton stands pre-eminent. 
The facts which most strongly countenance the undulatory theory of 
light are those derived from the phenomena of interference, phenomena 
which this theory alone can fully explain. These are, that rays of light 
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