234 Intelligence and Miscellaneous Articles. 



3. Bent armature and coils similar to No. 2. The magnet ver- 

 tical, and the revolving axis carrying the armature and coils at right 

 angles to the plane of the magnet. 



4. Similar to No. 3, with the exception of a second piece of 

 armature, with its coils, revolving on the same axis on the opposite 

 side of the magnet. The greatest power is obtained when the pieces 

 of armature are placed at right angles to each other. 



5. By fixed coils on the two branches of a horse-shoe magnet, 

 and a short thick piece of soft iron revolving in front of the poles. 

 This is a very neat form. 



6. By four cylinders of soft iron, with their coils, permanently 

 fixed to the poles of the magnet, one on each side of each pole. 

 The excitation is carried on by a revolving piece, as in No. 5. 



My unio* directive discharger, which can be applied to any of the 

 above forms, is by far the most happy contrivance I have yet hit 

 upon in this class of apparatus. It consists principally of four or 

 more semicylindric pieces, properly attached to a revolving spindle. 



The mercury, which has hitherto held so distinguished and im- 

 portant a situation in the discharging part of magnetic electro- 

 meters, but which is a complete nuisance to the operator, I have, in 

 most processes, entirely dismissed, by the introduction of my newest 

 forms of discharger. 



Artillery Place, Woolwich, Aug. 10, 1835. 



ON A METHOD OF ASCERTAINING, APPROXIMATELY, THE 

 RADII OF CURVATURE OF UNEQUALLY CONVEX LENSES. BY 

 N. S. HEINEKEN. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



If the following simple practical method of ascertaining the radii 

 of curvature of convex lenses appears to you to be worth insertion 

 in the Philosophical Magazine, it is at your service. 



I am, Gentlemen, yours, &c, 



N. S. Heineken. 



In order to ascertain the radii of the curvatures of crossed or 

 unequally convex lenses when great accuracy is not required, the 

 following simple practical method may be used, provided the radii 

 are not great : let the end of a piece of black or red sealing-wax be 

 a little softened by holding it near a candle ; breathe slightly upon 

 the lens, press the softened wax upon it, and let it remain until 

 cold. Upon removing the wax, it will be found, of course, to have 

 taken the form of the lens ; it will, in fact, have become a kind of 

 mirror f whose concavity will correspond with the convexity of the 

 surface of the lens. This mirror will be sufficiently brilliant to 

 form the image of a candle or the sun, by reflection, upon a piece 

 of card or white paper held before it. Now, the distance between 

 the centre of this mirror and the image thus formed will be equal 

 to half the radius of its curvature, and consequently to half the 



