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70 THREE-DIMENSIONAL MOTION PICTURES tance. from the lens may be found by similar constructions as the ones used for the locating of the principal foci. The formation of the principal foci by a lens, be it conver gent or divergent, is the resultant of the characteristics of the lens called the index of refraction and of the curvatures of its faces. This effect can be expressed as the particular power of the lens to impart to the incident rays a certain amount of con vergence or divergence. It has been explained that the propagation of light takes place in the form of spherical waves. Spherical waves proceed ing from an infinite distance have a radius of such length that the wave front reaching the lens may be considered a plane. As soon as such plane wave-front reaches the first surface of a lens each point of this surface becomes a center of disturb ance and the refracted rays in their new path of propagation form a wave-front which has a curvature dependent upon the power of the lens. This power will be positive for convergent lenses and negative for divergent ones and thereby convergent lenses are called positive and designated by the sign plus (~f) while divergent lenses are called negative and are designated by the sign minus (—). IMAGE FORMATION BY PLANE MIRRORS A mirror is a surface possessing a high reflecting power. The rays of light emitted from an object placed in front of a mirror are reflected by its surface, following the laws of reflection, and the eye placed in front of the mirror in any position will receive these rays which, reflected by the surface, will enter its pupil. The eye will then see an image of the object as if the rays were coming from behind the mirror. The part of the science of optics which deals with the phe nomena of reflection and especially with the formation of-im ages by mirrors is called catoptrics, and an instrument which forms images by reflection is called a catoptric instrument. It is evident that the image of any and all points of the ob~