In this section, the following term is used with the meaning indicated: "variable" (as a noun) means a feature or property (e.g., a dimension, a physical condition such as temperature, a quality such as density or colour) which, in respect of a particular entity (e.g., an object, a quantity of a substance, a beam of light) and at a particular instant, is capable of being measured; the variable may change, so that its numerical expression may assume different values at different times, in different conditions or in individual cases, but may be constant in respect of a particular entity in certain conditions or for practical purposes (e.g., the length of a bar may be regarded as constant for many purposes). Attention is drawn to the definitions of terms or expressions used, appearing in the notes of several of the classes in this section, in particular those of "measuring" in class and "control" and "regulation" in class . Classification in this section may present more difficulty than in other sections, because the distinction between different fields of use rests to a considerable extent on differences in the intention of the user rather than on any constructional differences or differences in the manner of use, and because the subjects dealt with are often in effect systems or combinations, which have features or parts in common, rather than "things", which are readily distinguishable as a whole. For example, information (e.g., a set of figures) may be displayed for the purpose of education or advertising (), for enabling the result of a measurement to be known (), for signalling the information to a distant point or for giving information which has been signalled from a distant point (). The words used to describe the purpose depend on features that may be irrelevant to the form of the apparatus concerned, for example, such features as the desired effect on the person who sees the display, or whether the display is controlled from a remote point. Again, a device which responds to some change in a condition, e.g., in the pressure of a fluid, may be used, without modification of the device itself, to give information about the pressure () or about some other condition linked to the pressure (another subclass of class , e.g., for temperature), to make a record of the pressure or of its occurrence (), to give an alarm (), or to control another apparatus (). The classification scheme is intended to enable things of a similar nature (as indicated above) to be classified together. It is therefore particularly necessary for the real nature of any technical subject to be decided before it can be properly classified. In this class, the following expression is used with the meaning indicated: "optical" applies not only to visible light but also to ultra-violet or infra-red radiations. In this subclass, the following terms or expressions are used with the meanings indicated: "simple lens or prism" means a single lens or prism; "compound lens or prism" means an optical member, the constituents of which either are close together without air-space or (except in group ) are "in broken contact", i.e. with the air-space between the constituents having no essential optical influence; "objective" means a lens or an optical system designed to produce a real image of a real object; "eyepiece" means a lens or an optical system designed to produce a virtual image for viewing by the eye or by another optical system; "front" or "rear" is determined by looking from the more distant conjugate. Attention is drawn to the Notes following the titles of class and subclass relating to "micro-structural devices" and "micro-structural systems". OPTICAL ELEMENTS Characterised by their structure: lenses; light guides; other elements ; ; Characterised by the material OPTICAL SYSTEMS General structure: number and arrangements of optical components , Special structures: according to purpose; with variable magnification; with reflecting surfaces ; ; Other systems STRUCTURAL DETAILS OF ARRANGEMENTS COMPRISING LIGHT GUIDES AND OTHER OPTICAL ELEMENTS OPTICAL APPARATUS Condensers Microscopes Telescopes, periscopes, instruments for viewing the inside of hollow bodies, viewfinders, aiming or sighting devices Eyepieces, magnifying glasses Other apparatus CONTROL OF LIGHT MOUNTINGS, ADJUSTING MEANS, LIGHT-TIGHT CONNECTIONS This subclass covers also contact lenses for the eyes, monocles, pince-nez, or lorgnettes. OPTICAL PARTS NON-OPTICAL PARTS Supporting arrangements; adjuncts , ; ATTACHMENTS OF OPTICAL PARTS TO NON-OPTICAL PARTS Principal; auxiliary ; ASSEMBLING, REPAIRING, CLEANING