In the majority of cases, the selection of one's first lens means the purchase of a camera, complete and ready for use, but this scarcely limits the scope for choice, for every grade of lens may be had today on the moderate priced cameras of all makers, and only in the cheapest styles is there no choice of lens equipment. We will begin with a consideration of the simplest forms of lenses, but we must first con sider the elementary principles of optics. Light and Its Transmission. To the physicist, light presents itself as vibrations of the luminiferous ether, a perfectly elastic medium which is supposed to fill space, and the principles of optics are deduced from a mathematical consideration of the changes of direction and velocity of these waves or Vibrations when passing from one medium to another. A much simpler conception is to assume that every luminous point emits rays of light which travel in absolutely straight lines in every possible direction as long as they remain in the same medium. In passing from a rarer medium, such as air, to a denser one, such as glass, the speed of the rays is lessened, and from this fact follows the possibility of a lens, as will appear later.