TRANSACTIONS OF SECTION A. 521 



a concave cylindrical lens varying in concavity at different parts according to the 

 •diameter of the cone at the corresponding part. Two such lenses mounted with 

 axes parallel and with curvatures varying in opposite directions produce a com- 

 pound cylindrical lens whose refraction in the direction of the axes is zero, and 

 whose refraction in the meridian at right angles to this is at any point the sum of 

 the refractions of the two lenses. This sum is nearly constant for a considerable 

 distance along the axis so long as the same position of the lenses is maintained. 

 If the lenses be slid one over the other in the direction of their axes, this sum 

 changes, and we have a varying cylindrical lens. The lens is graduated by marking 

 on the frame the relative position of the lenses when cylindrical lenses of known 

 power are neutralised. 



It was found by a practical optician to be impossible to work glasses on a cone 

 of large diameter, consequently a conical tool was constructed with an angle of 

 45° at the apex, and 8 inches diameter at the base, 



A glass about 4 inches long was ground on the sides of this near the base, and 

 as the resulting lens if ground on plane glass would have been too concave for most 

 purposes, the outer side of the glass was previously ground to a convex cylindrical 

 •curve, and its axis applied parallel to the generating line of the cone in the plane 

 of the axis of the cone. 



The result was concavo-convex cylinders of varying power suitable for the 

 practical measurement of astigmatism. 



Lenses were exhibited varying from to — 6DCy, and from to + 6DCy. 



7. On the Latv of the Propagation of Light. 

 By Professor J. H. Potnting, M.A., and B. F. J. Love, B.A. 



The authors describe a new method of proving that the intensity of illumina- 

 tion of a screen varies inversely as the square of the distance from the source. The 

 same experimental arrangement with slight modification allows us also to prove the 

 law of absorption of light — a law which we believe has hitherto been assumed 

 without veritication by experiment. The arrangement is as follows : — Two illumi- 

 nating surfaces at different distances are viewed through a narrow blackened tube, 

 each surface occupying half the field of view. The illuminating powers of the two 

 surfaces are adjusted till for a given distance of the tube they appear equally 

 "bright. They then appear equally bright for any other distance of the tube. 

 Tills was verified both for air and for an absorbing medium consisting of a dilute 

 alkaline solution of phenol-phthalein, which coloured the transmitted light violet. 



From this it can be shown that if I be the intensity of illumination of a screen 



I e~"^'^"" 

 at distance 1 from the source, the Ulumination at a distance x is When 



c = the medium is transparent. When c differs from 0, e-'^ is the ' coefB.cient of 

 absorption.' 



8. On a new form of Current-weigher for the Absolute Determination of the 

 Strength of an Electric Current. By Professor James Bltth. 



The object of this paper is to describe a method of absolutely determining the 

 strength of an electric current by measuring in grammes' weight the electro-magnetic 

 force between two parallel circular circuits, each carrying the same current. 



For convenience of calculation the circles have the same radius, and are placed 

 with their planes horizontal. 



The construction of the instrument is as follows : — A delicate chemical balance 

 is provided, and the scale-pans replaced by two suspended coils of wire. Each of 

 these is made of a single turn of insulated copper wire (No. 16 about) fixed in a 

 groove round the edge of an annular disc of glass or brass of suitable diameter. 

 The disc is made as thin and light as possible consistent with perfect rigidity. By 

 means of two vertical pillars of brass this aunulus is attached to a rigid cross-bar 

 of dry wood or vulcanite, in the middle of which is placed a hook for suspending 



