Societies and Academies

Abstract

LONDON. Optical Society, October 10.—Prof. Cheshire, president, in the chair.—T. Y. Baker: Sources and magnitude of centring errors in a sextant. A centring error in a sextant is ordinarily due to the sextant being placed eccentrically on the dividing engine. In order to comply with the National Physical Laboratory's “A”class certificate, it is necessary that this eccentricity should not be such as to produce errors in the reading exceeding 40 seconds. This condition is satisfied provided the scale-centre lies within a certain ellipse the centre of which is the mechanical centre of the instrument, and the axes of which lie one along and the other at right angles to the line of the middle reading. The semi-axes of this ellipse for a 7-in. sextant reading up to 120 are 5.2 mils and 0.7 mil respectively, but the former figure needs reduction to about 3 mils in order to allow of the vernier not reading “long” at the two ends of the scale. The customary practice of sextant-makers has been to readjust the position of the mechanical centre after the instrument has had the scale engraved. The workshop method of testing whether such readjustment is necessary is customarily the method of trying the length of the vernier against the scale at different points along the arc. The author showed that this method is not a sufficiently delicate test for the purpose of complying with the “A” certificate. An alternative method was described, in which the correctness or otherwise of the centre is determined by the tracing of a mark engraved upon the vernier against a circular arc cut from the same centre and at the same time as the marking of the scale. This method is being adopted by the Admiralty, and is already embodied in their specification for cadets' sextants.— T. Chaundy: Astigmatism: interchangeability of stop and obiect. For an object at O and a stop at S on the axis of an optical instrument, the astigmatism (i.e. astigmatic separation divided by the square of the height of the object) is to least order μ(I-FO.FS/f²) /μ′.SO, together with a quantity symmetrical in O and S. The planes of stop and object may thus be interchanged without change in value of the astigmatism if FO.FS =f². In this case, with like end-media, F′, S′, O′ (the images of F, S, O in the instrument) are symmetrically placed with respect to F, O, S. In particular, an object at one focus and a stop at the other are interchangeable. The astigmatism in this case is unaltered by reversal of the instrument; its consequent convenience in calculation is pointed out. In particular, all the primary aberrations may be determined by differentiation of its expression in terms of the powers and separations of the system.