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With Millimeter Measurements
Bibliographers in the former case will be undertaking original measurements directly from books and should heed the following advice. Twenty-line measurements should only be based on lines set solid. Unfortunately, it is not always possible to know whether there is leading between the lines. A convenient test for the presence of leading is to measure the interline spacing between a descender in the top line and an ascender in the next line down. A measurement between .5 and 1.0 millimeter usually suggests that the text is set solid.
Comparisons with other work may be complicated by the fact that bibliographers have not taken their measurements in a consistent manner. Some variation occurs because of ink squash—perhaps as much as one millimeter on the top and bottom of the sample. Thus, the use of a 6-10X power glass is recommended to determine the border of the type impression (Barker Interview). Additional variation can enter when one measures paper that is cockled or that has stretched or shrunk. In measurements taken for the Catalogue of Books Printed in the XVth Century Now in the British Museum (1908), for instance, A. W. Pollard notes up to a two per cent deviation ("Introduction," p. xx). Likewise, Philip Gaskell reports that "Paper shrinkage, which was more pronounced across the chain-lines than along them, generally reduced its dimensions by about 1 per cent, and occasionally by as much as 2½ per cent" (New Introduction to Bibliography [1972; repr. with corr. 1974], p. 13). These differences can be minimized by taking measurements on several different leaves.[4]
More importantly, Proctor measured "from the top of the short letters in line 1 to the bottom of the short letters in line 20, no account being taken of the tails on the longer letters."[5] As a consequence, his calculations are off by a fraction of an inch, as are measurements from the top of an ascender (e.g., h or l) in the first line to the bottom of a descender (e.g., g or y) in the twentieth line. The most accurate way is to begin any place in line one and measure to the same spot in line twenty-one. Any twenty-line measurement can be checked by measuring the type-page height and multiplying this figure by twenty and then dividing the product by the number of lines. This result should closely approximate the twenty-line measurement, although because the text type page measurement is usually taken to be the height from the text's highest ascender to its lowest descender, the result of the formula will not be precisely accurate.
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