. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. tor is somewhat important. Thebest possible is one not quite of a parabolic shape. Ifquite parabolic, and if the light were placed at thefocus, the rays would emerge in a parallel beam, andwould only give a small disc of illu-mination on the copy-board. But bymaking the reflector a combination ofa sphere and a parabola, and placingthe light nearer in than the true focus,a divergent beam is obtained, sub-tending an angle of nearly go°, thecopy-board being consequently veryevenly illuminated.
. The half-tone process. A practical manual of photo-engraving in half-tone on zinc, copper, and brass. tor is somewhat important. Thebest possible is one not quite of a parabolic shape. Ifquite parabolic, and if the light were placed at thefocus, the rays would emerge in a parallel beam, andwould only give a small disc of illu-mination on the copy-board. But bymaking the reflector a combination ofa sphere and a parabola, and placingthe light nearer in than the true focus,a divergent beam is obtained, sub-tending an angle of nearly go°, thecopy-board being consequently veryevenly illuminated. Messrs. Penroseand Co. first introduced these semi-parabolic—as we may call them—reflectors, and have been very success-ful in adapting them to arc lampsof various patterns (figs. 62 and 63).There is no doubt, also, a great gainin concentrating the light and throwingit forward to the copy-board, insteadof merely screening the back of thelamp with a sheet of tin or zinc, as was done , the latter plan is better than nothing at all, asit prevents stray light getting into the Fig. 62. THE ELECTRIC LIGHT INSTALLATION. 127 Whatever reflector is used it must be dead whiteinside. In most cases a coat of whitewash serves verywell. Bright reflecting sur-faces do not illuminate thecopy evenly. In regard to the choiceof lamps, some care mustbe exercised to get a suit-able type. If left to theelectrical contractor he mayput in a form which maybe very well suited for streetor general lighting, but notfitted to the needs of theprocess worker. The lampitself may not have beenmade to carry so heavy acurrent as 25 amperes,street lamps generally run-ning about 10 to 15 amperes,and although the lamp maybe adapted by changing the coils and carbon holders,it is never so satisfactory as a lamp designed for thecurrent it is intended to carry. Again, the street lampsare not expected to be so sensitive in feeding the car-bons together as they are consumed away, and in somecas
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