=373. Spherical Aberration.= Sometimes in a concave mirror when the aperture _MCN_ (Fig. 364) is large the images are blurred or indistinct. This is due to the fact that the incident rays near the outer edge of the mirror do not focus after reflection at the same point as those which pass into the mirror near the vertex, but cross the principal axis at points between the mirror and principal focus as is shown in Fig. 364; this result is called _spherical aberration_. The larger the aperture of the mirror the more the image is blurred. Concave mirrors in practical use do not have an aperture much greater than 10 degrees. This non-focusing of the rays of light by curved reflecting surfaces may be noticed in many places, as when light is reflected from the inside of a cup that contains milk or from the inside of a wide gold ring placed on top of a piece of white paper. The pupil will note other instances. This curve of light observed is called the _caustic by reflection_. =374. Parabolic Mirrors.=--The best possible surface to give to concave mirrors is parabolic. This is a curve which may be generated by moving a point so that its distance from a fixed point and a fixed line are always equal. If a source of light is placed at _F_ the rays after reflection are rendered parallel. See Fig. 365. This reflector is used in automobile lamps, headlights of locomotives, search-lights, etc. It is also used in large reflecting astronomical telescopes to collect as large an amount of light as possible from distant stars and bring it to a focus. Such mirrors may be made exceedingly accurate. [Illustration: FIG. 365.--Parabolic mirror.] Important Topics 1. Reflection: regular, diffused; plane mirrors; laws of reflection. 2. Formation and location of images by plane mirrors. Wave and ray diagrams. 3. Multiple reflection, illusions. 4. Curved mirrors, uses; concave, convex, parabolic. Exercises 1. Distinguish between regular and diffused reflection. By means of which do we see non-luminous bodies? 2. Could a perfect reflecting surface be seen? Explain. 3. A pencil is stood upright in front of a plane mirror set at an angle of 45 degrees to the vertical. Shown by a diagram the location and position of the image. 4. Show by diagrams the position and location of the images of a pencil (a) when standing erect and in front of a _vertical_ mirror. (b) when standing upon a horizontal mirror. 5. What is the difference between a real and a virtual image? 6. A standard candle and a lamp give equal illuminations to a screen that is 1 ft. from the candle and 6 ft. from the lamp. What is the candle power of the lamp? Explain. 7. Why are walls finished in rough plaster or painted with soft tones without gloss better for schoolrooms than glossy paints or smooth white plaster?