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Tuatara: Volume 18, Issue 3, December 1970

The Optical Coincidence System of Indexing Information

The Optical Coincidence System of Indexing Information


The Problem of keeping information from books and papers available for easy reference is familiar to specialists in most fields, and most are familiar with one indexing system or another which helps with the problem. The better known systems, however, have various shortcomings. They may require for their maintenance more time and concentration than is justified, or they may fail to yield information for want of recollection of some key word, or they may not be versatile enough to accommodate changing needs. Computers suffer from the first disadvantage, the alphabetical card (library) catalogue suffers from the first or second or both, and the edge-punched card index from the first and third.

A system which has none of these disadvantages and which is cheap to operate is the ‘optical coincidence’ system. It provides a cumulative index to information on any subject whatsoever; it provides with little effort, an unlimited degree of cross-indexing and it can be operated by untrained personnel of average intelligence. It employs standard index cards (5 × 3 inch or 8 × 5 inch) which need not be specially printed, though printed cards are easier to interpret.

Operation of an Optical Coincidence Index System

The equipment needed for the operation of the system is illustrated in figure 2. A wooden frame is constructed so that index cards of the chosen size fit accurately into it. A metal guide sheet the same size as the cards is cut from perforated copper or zinc, and painted or etched in such a way that the holes in it can be identified as a numerical series reading from left to right along the rows.

page 98
Fig. 1: A Printed Index Card

Fig. 1: A Printed Index Card

Items to be indexed (reprints, notes, specimens, etc.) are numbered consecutively in whatever order they come to hand. Suppose item number 1 is an article entitled ‘Blood Meal Identification in Aedes notoscriptus’ by M. Foot. The words ‘blood’, ‘Aedes notoscriptus’, ‘Foot, M.’, and other such relevant headings as ‘immunodiffusion’, ‘serology’, ‘Culicidae’, are each written on a plain index card (or a printed card, fig. 1), the cards are arranged face-up and stacked in the wooden frame. The metal guide-sheet is then placed on top of the cards, and a hole is drilled through the guide sheet perforation number 1, and on through every card in the stack. The cards are then page 99 removed from the frame and arranged alphabetically in a catalogue drawer. The hole in each card now enables it to be used to trace item number 1, and the cards therefore form the beginning of a general index to the information contained in the articles concerned. The next item is then processed in a similar way, significant words from the title and contents being written on cards which are then stacked in the frame and with the aid of the metal guide sheet, drilled through perforation number 2, and added to the alphabetical index.

As further items are processed, the card index grows and comes to contain an increasing variety of words. It then becomes possible to index later items by re-using cards from the existing index, only writing new cards for words that the index does not yet contain. Many cards will soon have more than one perforation, and some cards which are applicable to many items will carry many perforations. Such cards as ‘New Zealand’, ‘technique’ and ‘survey’ might be of this kind.

Information Retrieval

At its simplest the retrieval of information involves selecting some card from the index, interpreting (with the aid of the metal guide sheet) the numbers represented by its perforations, and consulting the items to which these numbers refer. Thus in the example cited above the card ‘Aedes notoscriptus’ might have three holes, say in the locations 1, 749 and 1305, and the three articles carrying those serial numbers will all have information on that species.

If, however, the card referred to has many holes, the task of consulting all the items concerned becomes excessively laborious. Say, for example, the card ‘New Zealand’ was selected and that it contained 470 perforations. The forbidding task of consulting all these items can be reduced immediately if a second relevant card, say ‘immunodiffusion’ is taken, placed exactly on top of the first, and the two cards held up to the light. Light will then be seen through only those holes which are common to both cards (i.e. are optically coincident) and these holes will refer to all the articles concerned with immunodiffusion and with New Zealand. If the number of articles is still too great for convenience, a third card, say ‘Canis canis’, may be added to the other two, and the optically coincident holes in the three cards will then refer only to articles concerning immunodiffusion, dogs and New Zealand. The number of such articles would perhaps be more manageable. It is this capacity for successive refinement of the quest for sources of information which is the special virtue of the optical coincidence method.

Technical Aspects of the System

The mechanics of the optical coincidence method may be handled in a variety of ways. For instance the metal guide sheet mentioned page 100
Fig. 2: A metal guide sheet and two unprinted index cards ready to be placed in the wooden frame for drilling.

Fig. 2: A metal guide sheet and two unprinted index cards ready to be placed in the wooden frame for drilling.

page 101 above, may be replaced or supplemented by printed index cards on which the locations of the perforations in the guide sheet are marked and numbered (fig. 1). Any index card marked in this way could serve as a master card either for the purpose of locating the hole to be drilled, or for interpreting the meaning of holes already made. The metal guide sheet has some advantages, however, as it constitutes a rigid guide for the drill, and it can be used to apply pressure to the stack of cards during drilling, which results in a cleaner perforation. The best technique seems to be the use of both guide sheet and marked index cards, for the guide sheet on its own can be clumsy in the process of information retrieval.

If holes are drilled out thoroughly, ordinary metalworking drills are quite satisfactory provided the stack of cards is compressed during drilling. A heavy paperweight is a help. The drill should not be less than 1/16 inch diameter if ordinary cardboard cards are used, nor should it be larger than 5/64 inch diameter if the cards are to accommodate 100 perforations to the square inch. With ordinary cardboard there can still be some tendency for adjacent perforations to coalesce, and there is a considerable advantage in using plastic-impregnated cards.

In filing the index cards alphabetically, the usual guide cards or tags are used to aid rapid location of the card required, and markers may be used to facilitate the return of cards to the index.

During indexing of the first fifty or so items a good deal of time is spent in writing cards for the alphabetical index, but as the index grows, more and more of the appropriate cards will be found already in the index so the effort involved in maintaining the system becomes less as the index progresses. Similarly the number of cards in the index is soon overtaken by the number of items filed. One such system, which contained 8,000 reprints, used only 1,500 index cards.

The number of cards that can be used in reference to a single item is actually unlimited. Even the length of the drill imposes no limitation as the cards can be drilled in several lots. (When drilling deep stacks, care must be taken to keep the drill upright.) It is therefore possible to index review articles and monographs under every species mentioned in the text, and for no more effort than writing the name of each species on a card. Cross reference on such a scale is a major advantage of optical coincidence over other systems.

Since every card in the index can be used as a template for drilling any number of duplicates of itself, the whole index can be reproduced quickly and accurately for the use of substations or agencies which have access to a central reprint collection, library or other information source. If it were desirable, every member of a research institution could be issued with a ready-made index to the contents of the library.

Though the capacity of each card is considerable (Miller's Bibliography of New Zealand Entomology’, which contains about 3,400 references, could be accommodated on 8 inch × 5 inch cards), page 102 there will come a time for most institutions when the capacity is exhausted. It is then necessary to close off the completed index and begin a new one, in much the same way as abstracting journals begin a new volume. In using the index for information retrieval, it then becomes necessary to consult both the old and the new index but this is not a very serious limitation in an index which gives access to the contents of 15,000 or 20,000 items. Ten inch × 10 inch cards will accommodate about 10,000 items; 5 × 3 inch cards, about 1,325.

Optical coincidence is a cheap, effective and manageable system which provides a cumulative cross-referenced index to information on any subject. There can be few specialists who would not benefit from using it.