PHOTOGRAPHY 



153 



Pltuto-lithogmphy and Zincography. The only 

 difference between these is that a lithographic 

 stone is used in the one case and a plate of zinc in 

 the other for the mechanical printing. It is neces- 

 sary that the original drawings should be done in 

 lines and not in Tialf-tint. At least, it is doubtful 

 whether much success has as yet attended the pro- 

 duction of half-tint photo-lithographs. A negative 

 is taken from the drawing by the camera, and from 

 it a print is made on paper coated with bichro- 

 matiscd gelatine much in the same way as in the 

 autotype process. But lief ore the print is devel- 

 oped it receives a coating of lithographic transfer 

 ink specially prepared for the purpose. It is next 

 floated in warm water till the lines are seen as 

 depressions. With the aid of a sponge and water 

 at a temperature of about 150 I . the soluble 

 portion is removed, leaving the picture in insoluble 

 gelatine with it* coating of transfer ink. It now 

 only requires to be washed, dried, and trans- 

 ferred to the stone or zinc plate (see LITIIO- 

 lili.U'HY). These processes have been greatly 

 superseded by the various photo-engraving and 

 zinc processes (fully descrilied !n the article ILLUS- 

 TRATION OK Books), by which printing blocks 

 suitable for a typographic press can be produced in 

 a few hours. 



Photography is now much employed as a means 

 of reproducing drawings on wood blocks for the 

 engraver. This process is of much importance, as 

 the original drawing is preserved, not only for 

 comparison with the finished engraving, but it may 

 be for its artistic value. The onginal drawing also 

 may be made of any convenient size, and reduced 

 on the wood a great consideration when minute 

 objects are to be represented. The necessities of 

 wood -engraving require that the drawing-on-wood 

 should be reversed ; hence the necessity of a re- 

 v,,,.,| negative in any direct printing process. The 

 negative may either be printed direct on to the 

 WIXM|, or a nuxlilication of the carbon process em- 

 ployed. In the first case, one process is first to 

 render the surface of the block waterproof, and 

 then it is whitened with Chinese white. The block 

 is then sensitised with chloride of silver, and printed 

 under a reversed negative. It is then toned with 

 gold and fixed with hyposulphite of soda, washed 

 mill dried, and is then ready for the engraver. 



In the carbon process, a carbon tissue is made 

 with very little gelatine and a large amount of 

 carbon or other pigment. The block is rendered 

 waterproof and whitened with baryta ; the carbon 

 print is develo|>ed on the wood with warm water, 

 and, when dry, is ready for the engraver. 



I'/tiilo-microaru/i/n/ ronsists in photographing 

 microscopic objects by causing a microscope to 

 take the place of the ordinary photographic lens 

 in tin- camera, so that the enlarged image is cast 

 upon the sensitive surface of the collodionisccl or 

 gelatine plate. Such photographs, again enlarged 

 by the optical lantern, are much used for class 

 instruction. 



By reversing the arrangement necessary for the 

 enlargement of microscopic objects it will lie seen 

 that minute photographs of engravings, or other 

 objects, may IK; produced which would require a 

 microscope for their inspection. In this way 

 communication was maintained during the invest- 

 ment of Paris, when copies of letters and news- 

 pajiers were inserted in quills, and fastened to 

 I'm lier pigeons. 



Avn:n\oMlCAL PHOTOGRAPHY. The applica- 

 tion of photography to astronomy has within the 

 past few years assumed immense importance, and 

 great results have been achieved through the wed- 

 iiiiL- '>f the camera with the telescope. Until 

 times tin: only remarkable photographs of 

 the celestial bodies were those of the moon, which 



, were executed by Warren de la Rue, Rutherford, 

 and others. The moon being from its size and 

 brightness a comparatively easy body to photo- 

 graph, the old processes were sufficient for the 

 purpose, and most perfect results were obtained. 

 JaiiM'n and others have secured photogiaphs of the 

 1 sun which exhibit markings upon its surface with 

 great distinctness, and many photographs of the 

 corona when the orb has been under eclipse have 

 been taken when the somewhat rare opportunity 

 has occurred. But it is in picturing the distant 

 stars and nebula- that the greatest work has been 

 achieved by photography, and results obtained 

 which would have been impossible without the 

 aid of the highly sensitive dry plates now at the 

 , disposal of the astronomer. Among the triumphs 

 i already obtained in this direction may be men- 

 ; tioned Roberts' photograph of the 'Andromeda 

 j Nebula,' Common's photograph of the Nebula in 

 Orion, and several similar negatives obtained by 

 the brothers Henry of Paris. One by these last 

 workers, a photograph of the Pleiades, should 

 receive special mention. A certain star in this 

 well known group appeared in the photograph in 

 question witli a nebulous haze attached to it. 

 This star was not known to be associated with a 

 nebula, and the astronomers of Paris in vain en- 

 deavoured to detect it by aid of the most powerful 

 ; telescope at their disposal. The nebulous mass 

 was therefore discredited, until another photo- 

 graph of the Pleiades arrived shortly afterwards 

 from America which exhibited exactly the same 

 peculiarity. Once more the nebula was searched 

 for, and at length was declared to be faintly dis- 

 cernible. From this it would seem that the 

 photographic Him is more sensitive to faint im- 

 pressions than is the retina of the eye, and in a 

 certain sense this is true. These star photographs 

 ! are often exposed to the action of the light from 

 those distant l>odies for three or four hours, during 

 which time the clock-work train attached to the 

 j telescope keeps the images of the tiny points of 

 i light stationary on the mate, in opposition to the 

 1 rotation of the earth. Each image, however faint, 

 has therefore a comparatively long time to make 

 an impression on the sensitive chemical surface, and 

 exerts a cumulative action, with the result that the 

 images of stars are registered which no human eye 

 has ever lieheld. To put the matter more plainly, 

 it may l>e said that a certain section of the sky 

 covered by the field of a powerful telescope is seen 

 to contain a definite number of stars. When this 

 same space is photographed their number is often 

 donbleil. At a convention of astronomers held 

 in Paris in 1887 it was decided to take steps for 

 photographing the whole of the heavens. For this 

 purpose the sky has been chartered out into squares, 

 and each observatory helping in the work will 

 photograph a certain number of these spaces. The 

 work is estimated to entail ten years' labour, this 

 long time being partly accounted for by the circum- 

 stance that, owing to the occurrence of unsuitable 

 weather and the interruptions caused by moon- 

 light, there are only about fifty nights in the year 

 when sidereal photography Ls possible. There are 

 many difficulties in photographing the stars, some 

 of which have led to discussion, and have caused 

 doubts to lie raised as to the accuracy of the results 

 attained. The chief of these is represented by the 

 circumstance that the photographs exhibit discs of 

 light, varying in size according to the brightness of 

 the stars, instead of mere points of light, which the 

 extreme distance of the l>odies should secure. This 

 expansion of size is believed to be due to irradia- 

 tion, want of complete achromatism in the lenses, 

 and reflection from the back of the photographic 

 plate. These difficulties will no doubt be sur- 

 mounted in the future, and it may bo mentioned 



