General and practical optics / by Lionel Laurance.

  • Laurance, Lionel.
Date:
1914
Catalogue details

Licence: In copyright

Credit: General and practical optics / by Lionel Laurance. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

    159/390 (page 149)
    vertical, and assigning its position accordinglj^; or it is done by simply deducting the numerical position of the axis from 180°. Thus, suppose the axis is at 60°, this is 30° to the right of the vertical; on turning the lens the axis is at 90° + 30° = 120°, i.e., 30° to the left of the vertical, or more simply by ISO - 60° = 120°. The corresponding positions of the axis as the one face or the other is in front are shown in the following table : 1st Position .. 0° 10 20 30° 40° 50° 60 70° 80° 90° When reversed .. 180° 170° 160° 150° 140° 130° 120° 110° 100° 90° Prisms.—The Base-Apex Plane.—As a prism, a sphero-prism, or a decentered sph. is rotated in front of the analyser, it is found that, in a certain position, there is a conUmiity of one of the lines tvithin and beyond the edges of the glass, as in Fig. 135, where the vertical line A B is continuous. The direction of this line indicates that of the base-apex plane of the prism, or of the prismatic element of the lens. If the Hor. line CD is deflected upwards, as to E F, the apex is then pointing upwards towards A, and the A B Fig. 135. base is down towards B. If the deflection of CD is downwards towards GII the edge of the prism is pointing downwards, and the base is up. If properly marked, the indicating scratches of circular trial prisms lie over A B when that line appears unbroken by the prism. Neutralisation of Prisms.—The strength of a prism can be learnt by neutralisation. The base apex line being located, the displacement of a bar of the analyser can be neutralised by trying one prism after another from the test case and placing it in opposition to the unknown prism ; that is, placing the base of the former over the edge of the latter, until that test prism is found which causes the bar to be seen continuous beyond and through the two prisms. The number of the test prism, which neutralises the unknown prism, indicates the A^alue of the latter. By this method the deviating angle is really neutralised, although the neutraliser may ])e numbered according to its principal angle. If the prism is combined with a sph. (or a sph.-cyl.) this latter must Ije first neutralised. With the lens and the neutraliser held together, the two