342 



THE CENTURY BOOK OF FACTS. 



radiated from them again. It is probable that 

 all bodies possess the quality in a greater or 

 less degree ; but with the great majority the 

 duration of the phenomenon is very short 

 rarely more than a small fraction of a second. 

 The phenomenon has no connection with or- 

 dinary phosphorus, but the name is thence 

 derived from the similar light emitted by phos- 

 phorus in the dark, which is due to the slow 

 combustion of this element, which oxidizes at 

 a very low temperature. 



Photography. The action of light on 

 chloride of silver was known as early as the 

 sixteenth century. The phenomenon was 

 studied by Scheele (1777), Senebier (1790), Hit- 

 ter and Wollaston (1801). From the results 

 of these investigations, experiments were 

 made by Thomas Wedgwood and Humphry 

 Davy, which were published, 1802. Wedg- 

 wood may be regarded as the first photog- 

 rapher. His paper was entitled " An Account 

 of a method of copying paintings upon glass, 

 and of making profiles by the agency of light 

 upon nitrate of silver." Further discoveries 

 were made by Niepce in 1814, and by Sir J. 

 Herschel in 1819. Daquewe commenced his 

 experiments in 1824; and in 1826 joined 

 Niepce, and worked with him till the death of 

 the latter in 1833. In 1839, Henry Fox Tal- 

 bot first published his mode of multiplying 

 photographic impressions, by producing a nega- 

 tive photograph (i. e., with the lights and 

 shades reversed), from which any number of 

 positive copies may be obtained. His patent 

 is dated February, 1841. From this time im- 

 provements have been made with great rapid- 

 dity. 



Celestial photography began with Professor 

 Bond, the astronomer, of Cambridge, Mass., 

 in 1851. It was greatly improved by Dr. 

 Draper in 1859 to 1881, and by others more 

 recently. 



Photogravure. The earliest attempt at 

 photographic engraving dates back to 1827, 

 which was six years previous to the introduc- 

 tion of the daguerreotype process, and was the 

 invention of M. Nicephore Niepce of Paris, 

 who first discovered that thin plates of bitu- 

 men were curiously affected by light. He 

 therefore coated metal plates with a thin layer 

 of bitumen of the kind called Jew's pitch, 

 and placed them in a camera obscura, so 

 arranged that he could insure their exposure to 

 the same image for several hours. The plate 

 was then submitted to the action of oil of spike, 

 which readily dissolved those portions not acted 

 upon by the light, but exerted little action 

 upon the remainder. The metal exposed by 

 the solution of the bitumen was then acted 

 upon by acid, which produced a complete 



etching plate, the picture part being protected 

 by its bituminous varnish from the action of 

 the acid. The art, which can now be per- 

 formed by several different methods, is also 

 known by the names of photo-zincography and 

 process-engraving. In ordinary zincography 

 the picture is laid, by the help of transfer 

 paper, on a zinc plate ; the parts to be pro- 

 tected are then covered with a varnish that 

 will resist acid, and the whole is then dipped 

 in a bath of dilute nitrous acid. This is re- 

 peated until the biting-in is sufficient, when the 

 plate is dried and the ink taken off with ben- 

 zine. In another process brass plates are 

 used, which are covered with white wax, the 

 design being drawn with an etching point 

 upon the wax. The plate is then submitted to 

 a powerful acid, which acts upon the parts of 

 the metal exposed by the lines, but does not 

 affect the wax. In photo-zincography the 

 drawing is photographed to the right size, and 

 an ordinary negative on glass is taken. This 

 is then laid on a sensitized zinc plate, on 

 which the picture is printed by the action of 

 light. The zinc is coated with bitumen, and 

 after the picture is printed, so much of the 

 bitumen as has not become insoluble by the 

 action of light is removed by a wash of tur- 

 pentine. In another process the photo- 

 graphic etching process the negative is 

 printed on a sensitized carbon paper, which is 

 then laid on a polished zinc plate, and, being 

 wet, all the carbon paper that does not hold 

 the lines of the drawing is readily removed. 

 The plate is then bitten-in in an acid bath. In 

 what is called the Ives process a negative is 

 applied to a gelatine plate, sensitized with 

 bichromate of potash. This plate is then put 

 into water, and all the parts not touched by 

 the negative will swell. A cast is then taken 

 of this in plaster of paris, which serves to form 

 a base for -electrotypes. The lines of engrav- 

 ing can also be reproduced by photography, 

 and a late process produces successfully intaglio 

 plates. Photo-engraving has enormously cheap- 

 ened the reproduction of pictures, but it does 

 not give plates that print with the clearness 

 and distinctness of those taken from wood 

 engravings. 



Physics has been described as a science of 

 unbounded extent, and as reaching from an 

 atom to God himself. It is made to embrace 

 the entire doctrine of the bodies and existences 

 of the Universe : their phenomena, causes, and 

 effects. Lockwood would include God, angels, 

 and spirits under this term. The origin of phys- 

 ics is referred to the Brahmans, magi, and 

 Hebrew and Egyptian priests. From these it 

 was passed to the Greek sages, particularly 

 Thales, who first professed the study of nature 



