Anyways, much of the later chapters are step-by-step through some individual preparations-- a unique approach that as a beginner I appreciate (even if some of the chemistry and methods are quite dated). It's a cheap used book, but also available for free on the Internet archive as a good facsimile edition:Two methods of designating the primary magnifications of objectives
are still in use. The obviously intelligent one, followed by most contem-
porary manufacturers, is to engrave the actual figure on the lens barrel.
The older method, still all too frequently seen, and given for comparison
in the table of magnifications, is to engrave the "equivalent focus." When
this equivalent focus is given in inches— e.g., 1/2 in., 2/3 in., 1/6 in.— it has
some actual meaning. The equivalence, in this case, is to the magnifica-
tion of a simple lens at a distance of 10 in. A simple lens of 1 in. focal
length will cast an image magnified ten times at a distance of 10 in.
Similarly a 1/2-in. lens will give a X20 image at the same distance, or a
2/3-in. lens a Xl5 image. It is therefore relatively easy, when a lens has
E.F. 1/6 in. engraved on the barrel, mentally to convert to X60. The
engraving of E.F. in terms of millimeters is a stupidity of Teutonic origin
that should never have originated and is unforgivable when used by
American manufacturers. To interpret, for example, E.F. 2 mm, it is first
necessary to convert 2 mm to y 12 in. before arriving at X120. Actually-
most so-called "2-mm" lenses have a primary magnification of X90,
which reduces the whole system to imbecility.
https://archive.org/details/handbookofbasicm1958gray
