IN these days when many of our great cities are involved in so much rebuilding, the question of light easements of adjoining property frequently gives rise to the necessity for financial settlements or building restrictions involving a definite assessment of rights of light existing. Speaking generally, the law recognises that light enjoyed over a sufficient period, so far as the use to which the particular space lighted is usually put, cannot be materially encroached upon without some form of compensation. At one time, cases in the courts depended on such general evidence as could be brought forward; more recently, geometrical methods have been worked out whereby the actual illumination can be measured. These methods are laborious, and Mr. A. S. E. Ackermann, 17 Victoria Street, London, S.W.I, sends us particulars of an invention for determining sill ratios which involves neither photographic nor photo metric work. It consists of a pane of clear glass attached to two adjustable radius bars, the whole mounted on a stand. This is set up to face the window in question with the centre of the glass at the middle of the sky area. The sine of the elevation is read and the sky area traced on the glass, the observer using a pinhole eyepiece. This diagram is transferred to tracing paper and the sky area measured by a planimeter. This area multiplied by the sine of the elevation angle and divided by a constant gives the sill ratio. The instrument is portable and weighs less than 9 Ib.