Rudolph Kingslake in A History of the Photographic Lens (1989): "It is certain that the Tessar was not a modified Cooke Triplet, as the series of steps followed by Dr. Paul Rudolph in going from the Anastigmat to the Tessar are well established, but for some of the later designs it is not always clear whether they should be regarded as modified Tessars or modified Triplets." About the Ross Xpres: "The Tessar was such an excellent design that other workers would have liked to copy it but were prevented from doing so by patent limitations. The simplest way out was to use a cemented triplet in the rear instead of a doublet. Several designs of this type appeared in 1913, including Ross' Xpres by J. Stuart and J. W. Hasselkus." (Source: medium-format-photography-forum)

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