A manual of photographic chemistry, theoretical and practical . point are brought to a focus by LENSllS. 331 means of a lens, so are they wlienthey proceed from an object,and in that case an image of the object is the result. In orler that the course pursued by pencils of rays proceed-ing from an object may be easily traced, the lines from thebarb of the arrow in Fig. 25 are dotted. Observe that the objectis necessarily inverted, and also that those rays which traversethe central point of the lens, or the centre of the axis, as it istermed, are not bent away, but pursue a course either coin-ci
A manual of photographic chemistry, theoretical and practical . point are brought to a focus by LENSllS. 331 means of a lens, so are they wlienthey proceed from an object,and in that case an image of the object is the result. In orler that the course pursued by pencils of rays proceed-ing from an object may be easily traced, the lines from thebarb of the arrow in Fig. 25 are dotted. Observe that the objectis necessarily inverted, and also that those rays which traversethe central point of the lens, or the centre of the axis, as it istermed, are not bent away, but pursue a course either coin-cident with, or parallel to, the original, as in the case of refract-ing media with parallel surfaces. Enlarged and Reduced Images.—When an object is placedat some distance in front of a lens, an image is formed smallerthan the object; but if the object be advanced nearer to thelens the image increases in size until it becomes, iirst equal to,and then larger than, the original, the focus at the same timereceding to a greater distance from the lens. /f\^^-:--. Fig. 25. Thus, if a photographic portrait be approximated to a lensan image of the life size may be thrown upon a screen, just asthe rays proceeding from the living model were in the first in-stance employed to form the portrait. The two planes, infact, of the object, and of the image, are strictly conjugate foci(page 396), and it is immaterial from which of the two, anterioror posterior, the rays of light proceed. Amplifying instruments, like Woodwards patent solarcamera are constructed upon the above principle, and arefound useful in enlarging photographs to three or four timestheir original diameter. A loss of definition always results inthis process, which may conveniently be remedied by the pencilof the artist. Lenses of Long and Short Focus.—A lens of short focusplaced at a given distance from an object forms a small image, 332 OPTICS OF PHOTOGKAPHY. the rays being strongly refracted. If the lens be broughtnearer t
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