In a paper under the above title (Journal, Vol. 1, p. 266) J. P. G. Worlledge has made the useful suggestion that sextant certificates should give a critical table for the corrections instead of the present practice of giving the corrections at a few equally spaced readings. Unfortunately the method which he gives of deriving the proposed form from the present form is unsound. The centring error is correctly stated as

where Z is the sextant reading (the difference between Z and the true angle in this formula is negligible). But part of the centring error is included in the index error, so that to obtain the part of the centring error shown on the certificate the value of C for Z= 0 must be subtracted. This yields

for the formula of the correction.