Levelling concepts – meaning of a central bubble in a level tube When the bubble of a level’s bubble tube is centered (under permanent adjustments), which statement correctly describes the resulting geometry?

Introduction / Context:
Levelling depends on the instrument’s internal relationships (permanent adjustments) that link the bubble axis, the vertical axis, and the line of collimation. Understanding what “bubble centered” guarantees is essential for correct interpretations of readings. This question asks which line is truly horizontal when the bubble is centered in a properly adjusted instrument.

Given Data / Assumptions:

• The instrument has been correctly adjusted: bubble axis is perpendicular to the vertical axis, and the line of collimation is parallel to the bubble axis.
• Bubble is observed to be central in its run.
• Standard levelling definitions apply.

Concept / Approach:

With permanent adjustments, centering the bubble aligns the bubble axis horizontally. Since the line of collimation is made parallel to the bubble axis during adjustment, the line of collimation is therefore horizontal whenever the bubble is centered. The terms “axis of the telescope” or “geometrical axis” can be ambiguous, whereas “line of collimation” is the precise line through the cross-hairs and optical center defining the sight line used for readings.

Step-by-Step Solution:

Verification / Alternative check:

Instrument manuals specify that in a properly adjusted level, the line of collimation becomes horizontal when the bubble is centered; two-peg tests and collimation checks verify this relationship empirically.

Why Other Options Are Wrong:

Axis of the telescope / geometrical axis: These are not the operative definitions in levelling; optical and mechanical axes can differ and are not guaranteed to be aligned to the bubble axis.

“Line of sight” (colloquial): Often used loosely; the precise term in levelling is “line of collimation.”

Common Pitfalls:

Using imprecise terminology; assuming bubble centering guarantees every conceivable axis is horizontal; neglecting the need for periodic permanent adjustment checks.

Final Answer:

Line of collimation is horizontal