Thorne Cartographic Illusion

Definition and scope

The Thorne Cartographic Illusion is a persistent visual phenomenon affecting floorplans, causing minor geometric inconsistencies upon a third review that are not present in the physical space. The inconsistency is conventionally described as "minor": a corridor shown as 40 centimetres longer than surveyed, a niche bank implying eleven units where ten are installed, a stair landing rendered with a chamfer that the masonry does not have. The discrepancy is not present on the first or second reading of the drawing. It appears on the third, and it remains thereafter for the life of the sheet.

The Illusion is catalogued in three registers, which the Librarians have never merged. As a phenomenon it is a property of drawings; as a transient Cartographic Inconsistency it is a reportable event with a duration and a witness; as a practice it is the routine work of archivists who must state, on the face of any sheet they certify, that the sheet may disagree with the room. The third sense is the one that generates the most correspondence.

Early documentation

The earliest surviving description is a marginal note on a floorplan of the south scriptoria, unsigned, dated only to the winter of a year recorded elsewhere as the year of the Ash Landing collapse. The note reads: "Third reading shows the alcove deeper. Measured twice, alcove is not deeper." Subsequent archivists treated this as an instruction rather than an observation, and for several decades it was standard practice to read every new floorplan twice and then set it aside for a day before the third reading, so that the discrepancy, when it arrived, would arrive in daylight.

The naming of the phenomenon is disputed. One lineage attributes it to Arin Thorne, whose prolonged administrative dispute with Arin Vex concerned cartographic cross-references and the Thorne Principle; another attributes it to Commissioner Aris Thorne, developer of the same-named principle, on the reasoning that a phenomenon involving provisional accuracy should carry the name of provisional permanence. A third attribution, recorded in the registry's dissenting annals, gives it to K. Thorne on the grounds that the Illusion, like the Thorne Progression, describes a logarithmic worsening — small on first inspection, larger on each inspection after. None of the three Thorne whose names are in contention appears to have claimed it during their tenure.

Explanatory theories

Structural Stress Theory, the position held by the Survey Office since the promulgation of the Sixth Concordance, posits that the Illusion is caused by sympathetic resonance between drafting materials and fluctuations in Structural Integrity Units. Under this account the drawing is not wrong; it is reporting a stress the room has not yet admitted. Sheets held on floors with high unit counts develop the inconsistency faster, and the theory's proponents point to the winter of the twelfth recension, when a run of floorplans for the north galleries each acquired the same 12-centimetre error within a span of nine days, all of them drafted on the same batch of vellum from the same stall.

The dissenting account, associated with the Cartographic Revision Bench, holds that the third review is simply the review at which the reader stops checking the drawing against the room and starts reading the drawing against itself. On this view the discrepancy is a defect of comparison, not of paper, and the fact that two readers handed the same sheet on the same afternoon have both reported the alcove as deeper is explained by the fact that the second reader was told what the first had found. Amendment 7-c to the Sixth Concordance, "On Representational Fidelity," declines to choose, and requires archivists only to acknowledge potential "Thorne-Type discrepancies" in any certified copy.

The two positions are not as far apart as the Concordances suppose, and both are embarrassed by the same case: a floorplan of the Thorne Ceiling maintenance corridor, reviewed on four separate occasions by four separate clerks, which produced four different alcove depths, none of them the surveyed figure.

Incident of the eleventh recension

The most consequential episode remains the survey of the eleventh recension, when a drafting error of the Illusion's standard type propagated into an installed partition. The partition stood 40 centimetres proud of its marked position for two years before a visiting clerk from Thorne v. The Registry noticed that the corridor beyond it could not be walked with a full bundle. The resulting correction required the Thorne Ceiling maintenance corps to be re-rostered for a fortnight, and the Concordance cited in the re-rostering was itself superseded before the work began, an instance of the Illusion of Concordance that is studied alongside the Illusion proper in the joint seminars held on the theory floors.

The Registry's own account of the incident attributes the error to a draughtsman; the draughtsman's guild attributes it to the sheet; the sheet, being an object, attributes nothing, and is retained in the evidence annex in the condition in which it was found, third-reading error and all.

"The room is surveyed. The drawing is surveyed. That one of them should lie is tolerable; that they should take turns is not."

— Sixth Concordance, Amendment 7-c, preamble

Standing practice

Current practice requires that any floorplan intended for use beyond its floor of origin be accompanied by a discrepancy schedule, and that the schedule be re-attached whenever the plan is recopied, since copying resets the reading count. It is this reset that accounts for the frequency with which the Illusion is reported as newly discovered: a copy of a sheet that has erred for a century reads clean twice more before erring again, and the archivist who finds it is not wrong to think they have found something.

Drawings held in the evidence annex of Thorne v. The Registry are exempt from recopying, on the grounds that their evidentiary value lies in their having erred. Visitors to the annex are permitted to read any sheet three times; a fourth reading requires the countersignature of the floor's Clerk, and has been refused on four recorded occasions, always on the same morning.

Readers intending to cite the Illusion are reminded that the floorplans of the survey corridors are unavailable between the hours of dusk and first light, as the drafting stalls in those corridors are used for other purposes at that time, and the sheets returned to their drawers still warm.