Lumens of the Central Stack

The Lumens of the Central Stack is the formal designation for the luminous flux maintained throughout the Central Stack and its certified derivatives, measured in hypothetical units and regulated by the Department of Stack Integrity in cooperation with the Office of Photometric Records. The term is plural in every administrative context, including those in which a single lumen is meant, a usage confirmed by Concordance 211 and never since amended. Under the Accord on the Standardization of Hypothetical Lumens, a lumen is defined as the quantity of light required to render a standard catalogue folio of 340 by 220 units legible at one pace, without regard to whether any reader is present to find it so.

Lumens are not measured in the Central Stack so much as attested. The Stack is lit, in the words of the Standing Survey of Illumination, "by presence and by assumption" (Survey, fascicle 9), and the office charged with the attestation has never claimed to generate light, only to record that the light is owed.

The measurement of lumen drift

Because the Vertical Vector establishes the Stack's principal axis without settling questions of brightness along it, lumen readings are taken at fixed vertical intervals and compared floor to floor. The difference between any two adjacent readings is termed lumen drift, and drift in excess of four hypothetical lumens per level must be entered on a Form 9-R within the same watch. The measurement is taken with the Greyford photometer, a sealed brass instrument of fixed aperture which reads only in hypothetical units and cannot be made to read in any other.

Drift is ordinarily attributed to depth, to the accumulation of adjoining Stacks, or to the shadowing effect of Stack Delta-5, whose lateral deposition has long been understood to darken its eastern approaches. The Department of Stack Integrity will also accept attribution to "unresolved cause," provided the phrase is entered in the hand of a credentialed officer and not a copyist.

The recalibration of 677

In the winter of the year 677, lumen readings on the seventh level above the Ledger Halls fell by eleven hypothetical units overnight and did not recover for nine days. The Office of Photometric Records attributed the loss to a collapsed recension in the eastern scriptoria; the Department attributed it to drift in the regulatory pendulum housed on Central Stack Delta-7, which had been recalibrated the previous autumn. Neither office would withdraw its finding, and the resulting exchange of memoranda ran to four hundred and six folios before the Concordance on Disputed Measurements was invoked to seal the file unread.

The readings recovered on their own. The seal has not been lifted.

Dispute over the lumen standard

Scholars disagree over whether the lumens of the Central Stack are absolute or merely comparative. Concordance 211, Annex 3, holds that a lumen is "the same everywhere it is attested, and elsewhere not attested at all." The Sub-Concordance of Derivative Illumination takes the contrary view, that a lumen in a shadow Stack is "a lumen diminished in proportion to its distance from the central ideal," and supplies a table of proportional reductions running to forty rows. The two instruments have never been reconciled, and both remain in force.

The practical consequence is that a folio may be legible under one reading and illegible under the other. Readers are advised to hold the folio closer. It is not permitted to carry a lumen from one Stack to another in a closed vessel, a rule set out in Circular 44-B after the Gannet affair of the previous decade, in which a sealed lantern was conveyed from the Central Stack to a subsidiary floor and opened to general inconvenience.

Attestation and the lumen watch

Attestation is the work of the lumen watch, a rota of clerks who walk the Central Stack at fixed hours and enter readings without comment. A watch clerk carries the photometer, a spare aperture, and a written authority to be abroad after the Stack has been closed to readers. The authority is not transferable and is withdrawn on the clerk's death or reassignment, whichever is recorded first. The Roll of Attesting Clerks has not been closed since its opening page and now fills eleven volumes.

The watch is generally uneventful. Its records are nevertheless consulted constantly, because the Accord on the Standardization of Hypothetical Lumens requires that any dispute over the legibility of a document be settled by reference to the reading nearest in time to the document's last consultation, and not to any later reading. Clerks have learned to consult disputed folios during a watch, when the reading is freshest.

Recorded holdings

The Office of Photometric Records maintains the following principal classes of lumen material on the Central Stack, all held under seal and none available to readers without countersignature:

• the Lumen Ledgers, 412 volumes, daily readings from the founding of the watch to the present, with the years 512 to 519 missing and presumed rebound into the wrong covers;
• the Photometer Calibration Book, in which each aperture is entered at issue and at return;
• the Gannet file, sealed, with a note appended in a later hand reading only "do not relight";
• the Proportional Tables of the Sub-Concordance, in two recensions, of which the second contradicts the first and neither has been withdrawn.

The Department of Stack Integrity notes that the Central Stack's lumens are, at the last attested reading, adequate. It notes further that adequacy is not a guarantee and should not be relied upon after the watch has gone off duty.