A New Mode of making Magic Mirrors. 547 



light reflected from the disk. These marks were found to 

 correspond to concave strains produced by hammering the 

 plate into rough convexity before scouring down with char- 

 coal. Depressions produced by electro-deposition of silver 

 on a silvered plate, protected in parts by varnish according to 

 the figure desired, were therefore tried ; but they invariably 

 deepened into the underlying brass before the sharp edges of 

 the figure were polished away. 



Next, a fairly thick coating of silver was deposited on the 

 plate, and a pointed bit of agate was repeatedly drawn with 

 pressure over certain parts of the porous layer of metal. The 

 figures thus obtained were startling ; for the depressions ap- 

 peared on the screen as reticulated lines of deep shade, which 

 had their analogue in the broken spinal divisions left by the 

 agate point. The bad working of metals electrically deposited 

 caused me to give up the deposition method. 



My plate of brass was next slung up in a weak solution of 

 copper sulphate and sulphuric acid on the positive wire of a 

 pint bichromate-cell. After an immersion of four minutes, 

 the parts of the plate not protected by the naphthaline solu- 

 tion of sealing-wax came out beautifully fretted. After 

 several attempts on these lines, and two days after the lecture 

 delivered here by Prof. S. P. Thompson on the 27th of 

 January last on the Magic Mirror, my first success was 

 achieved in the shape of a mirror 2 inches in diameter, 

 representing a stem with leaves and a guide-post standing in 

 a mound of earth. From that mirror to my present one, the 

 first of the new class being completed in the following July, 

 was, however, a far cry. Applied to larger mirrors and 

 figures, the electrical method proved essentially bad. Exactly 

 contrary to what was required, the figures were deeper at the 

 edges of the lines than at the central parts : indeed, a broad 

 line scoured down into two lines defining the lateral limits of 

 the original one. 



Nitric acid was finally adopted as the figure-eating agent, 

 care being taken to use good brass and to polish well before 

 immersion, so that the action on the plate might be uniform. 

 Smooth figures were thus produced after immersions of up- 

 wards of six seconds in strong acid. The walls of the figures 

 are perpendicular to the surface of the disk, and the deter- 

 mination of their proper height relatively to the breadth of 

 the lines of the figure involved much further labour. The 

 process of scouring with Sheffield lime and swans'- down 

 calico has a double effect — it rounds off the upper rectangular 

 edges of the walls, and at the same time converts the fiat 

 floors of the figure into concave depressions, the walls and 



