This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1902 edition. Excerpt: ...of the complete doublet, while the front combination has a focal length of about twice that of the complete doublet. This lens works at /6 and the size which is best adapted for general work on a plate 8 x 6i (10'7 ins. focal length) will cover a 15 x 12 plate if a stop of /16 is used, and this with fairly equal illumination. The number of anastigmatic doublets capable of division into two good landscape lenses is now so great that even the names cannot be mentioned. The above are, however, representative examples. 16 The pass test for an Anastigmatic or Stigmatic Doublet.--The long column of Times leader having been set up as already directed Fio. 102. and to the same standard of reduction (p. 84), it should be possible to find a focal plane in which the whole is distinctly legible at the same time, the full aperture of the lens being used. If the lens has a greater rapidity than /6 a little leniency may be allowed. For the whole-plate lens (about 11 ins. focal length) the racking in for curvature of the field should not exceed one tenth of an inch, and the astigmatic difference should not be more than one twentieth of an inch, the test being made as on the margin of a whole plate, as directed (p. 84) for the case of a rapid symmetrical. In the case of lenses of longer or shorter focal length proportionate errors may be allowed. The test for achromatism is made as already directed in the case of the symmetrical. The anastigmat should pass the test absolutely, but care must be taken not to misjudge by reason of want of register between dark slide and focussing frame. A very firm and well made camera is required for this test. 17 Combining Lenses: further Notes.--The lenses described above may be combined to form doublets, and when...