The careful use of cross references is next in importance to the selection of appropriate headings. Great judgment, however, is required, as the consulters are naturally irritated by being referred backwards and forwards, particularly in a large index. At the same time, if judiciously inserted, such references are a great help. Mr. Poole says, in an article on his own index in the _Library Journal_: "If every subject shall have cross references to its allies, the work will be mainly a book of cross references rather than an index of subjects." He then adds: "One correspondent gives fifty-eight cross references under Mental Philosophy, and fifty-eight more might be added just as appropriate."
The indexer should be careful that his cross references are real, but he has not always attended to this. In Eadie's _Dictionary of the Bible_ (1850) there is a reference, "Dorcas _see_ Tabitha," but there is no entry under Tabitha at all.
In Cobbett's _Woodlands_ there is a good specimen of backwards and forwards cross referencing. The author writes:
"Many years ago I wished to know whether I could raise birch trees from the _seed_.... I then looked into the great book of knowledge, the _Encyclopædia Britannica_; there I found in the general dictionary:
"'BIRCH TREE--See _Betula_ (Botany Index).'
"I hastened to BETULA with great eagerness, and there I found:
"That was all, and this was pretty encouragement."
William Morris used to make merry over the futility of some cross references. He was using a print of an old English manuscript which was full of notes in explanation of self-evident passages, but one difficult expression--_viz._ "The bung of a thrub chandler"--was left unexplained. In the index under Bung there was a reference to Thrub chandler, and under Thrub chandler another back to Bung. Still the lexicographers are unable to tell us what kind of a barrel a "thrub chandler" really was. I give this story on the authority of my friend, Mr. S. C. Cockerell.
No reference to the contents of a general heading which is without subdivision should be allowed unless of course the page is given.
There are too many vague cross references in the _Penny Cyclopædia_ where you are referred from the known to the unknown. If a general heading be divided into sections, and each of these be clearly defined, they should be cross referenced, but not otherwise. At present you may look for Pesth and be referred to Hungary, where probably there is much about Pesth, but you do not know where to look for it in the long article without some clue. Sometimes cross references are mere expedients, particularly in the case of a cyclopædia published in volumes or parts. Thus a writer agrees to contribute an article early in the alphabet, but it is not ready in time for the publication of the part, so a cross reference is inserted which sends the reader to a synonym later on in the alphabet. In certain cases this has been done two or three times. An instance occurs in the life of the distinguished bibliographer, the late Henry Bradshaw (than whom no one was more capable of producing a masterly article), who undertook to write on "Printing" in the _Encyclopædia Britannica_. When the time for publication arrived (1885), Bradshaw was not ready, and in place of the article appeared the cross reference, "PRINTING, TYPOGRAPHIC--See _Typography_." Bradshaw died on February 10, 1886, and the article on "Typography" which was published in Vol. 23 in 1888, was written by Mr. Hessels.