COLLIMATION, Line of. In a measur ing telescope there is placed at the focus of the eyepiece a system of spider-threads. In a tran sit Instrument there are several vertical and two horizontal °wires," as shown in the diagram.
In spite of numerous experiments in a search for something more durable than spider threads, are still almost universally formed of this fragile material. Quartz fibre and finely drawn platinum wire have at times been em ployed, but it is found that the fineness and evenness of threads from chosen varieties of spiders exceeds anything that can be produced by artificial means. In smaller instruments the wires are frequently fine lines drawn on a glass plate.
In taking an observation the time at which the star passes behind each wire is noted. Tak ing the mean of these times, we obtain what is called the time for the "'mean wire," an imagi nary wire which would, if the adjustments were perfect, coincide with the middle vertical wire. The mean of these observations gives a better result than if only one central wire were used. The line of collimation is defined to be the straight line that joins the centre of the object glass with the point of this imaginary vertical wire midway between the two horizontal wires. A definition similar to that here given applies in other cases, in the case of the mural circle, for example, and in the case of instruments for terrestrial surveying. The proper adjustment of the line of collimation of these instruments is one of the most important of the adjustments.
In the case of the transit instrument, for in stance, in observing the passage of a star, what is noted is practically the instant, according to the observatory clock, when the line joining the star and its image coincides with the line of collimation. The following is one of the condi tions to be satisfied when the instrument is in perfect adjustment. The line of collimation
must be perpendicular to the geometrical axis on which the telescope revolves, and will then describe a great circle. The framework that carries the spider-lines admits of several small movements for their adjustment. After the ad justment has been carefully made, however, there always remains a slight error, which is de termined and allowed for in calculations under the name of the collimation error. Adjustments and corrections are similarly made in the other telescopes for measuring.
When the central wire has been fully ad justed and placed in the meridian by the obser vation of stars, it is usual with larger instru ments to facilitate its future adjustment by the use of Collimators. These are telescopes mounted horizontally on piers, one north and one south of the instrument, and so arranged that when the latter is horizontal it will point directly into one of the collimator telescopes, which are made of the same aperture as that of the instrument itself. At the focus of each lens a spider thread is placed, which is thus viewed through the collimator exactly as if it were at an infinite distance away. Having ad justed the collimator threads in the meridian, the iniddle thread of the instrument can at any future time be placed upon them in a few sec onds, thus saving a great deal of time. Some times the collimator lenses are made of a very long focus, even so great as a few hundred feet. The wires, or artificial stars, are then placed upon piers in so-called Collimator Houses, or Mire Houses, one north and one south of the meridian house, and a few hun dred feet distant from it.