The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Light
colours, black; and one that absorbs none, white ; and so on. A ray of light is reflected from a polished surface at the same angle with a line drawn perpendicular to the surface of mirror, that the ray striking the mirror makes - i.e.> u the angle of incidence is equal to the angle of reflection." When a ray of light strikes a plane reflecting surface, it will, after reflection, seem to come from a point situated similarly behind the mirror - i.e., the image of an object 10 ft. away from the mirror will be reflected from a point seemingly 10 ft. behind the mirror: the eye in no way recognising the bending of a ray, but seeing as if the ray were straight, and corresponding in direction to that portion striking the eye. When light passes from one transparent medium to another transparent medium, unless perpendicular to the sur-faces of both, it is refracted or bent aside to a greater or less degree. This power of refraction differs for every different substance, but remains the same at all times for the same substance. It is expressed by the ratio of the sine of the angle of refraction to that of the angle of incidence. Thus from air to water it is 4 : 3. When a ray of light is refracted, it also suffers what is termed dispersion - i.e., it is separated into its constituent coloured rays. (See Decomposition of Light.) The following table given by Eder, and showing the relative intensity of various sources of light, must obviously be taken as very approximate; still it may be useful from a practical point
of view: -
Candles.*
Arc light from dynamo          ... ... 200 to 6,000.
,,         ,,         40 Grove's cells         ... 360.
,,         ,,         48 Bunsen's cells ... 380.
Lime light ... ... ... ... 90 to 790.
Magnesium wire          ... ... ... 100 to 200.
Oil lamp           ............ 10 to 11.
.. ,, fed with oxygen ... ... 60.
Gas flame         ...... ... ... 6 to 20.
Welsbach incandescent light... ... 60.
Sunlight           ............60,000.
Light of full moon ^7nn>TJ7 ^o innnroTr of sunlight. Very many interesting facts about light, especially in its rela-tions to electricity, might be here stated; but as nothing like
* For ratio of German candles to English candles see p. no. 451