518 A New Mode of making Magic Mirrors. 



floors finally merging into one concave sweep. This curve, 

 with narrow lines, is sharper than with broad lines; con- 

 sequently the latter may disappear, while the former remain 

 visible. Yery brief immersions in the acid are therefore not 

 suitable for figures having lines differing much in breadth. 

 Long immersions, on the other hand, are objectionable both 

 on the ground of the excessive labour in rounding off the 

 edges, and of the well-nigh impossible task of reducing 

 uniformly the deep runnels of the figure by scouring the 

 general surface with charcoal. 



Gradations in depth according to the breadth of the several 

 limbs were therefore tried, and produced in the following 

 way : — The figure, say of a tree, is made by removing paraffin 

 wax from the brass plate with a pointed stick of boxwood, and 

 is then fixed by a very short immersion in the acid. The 

 slender branches are now painted over with hot wax, and the 

 broader branches and trunk are reduced in breadth sym- 

 metrically by the same means, the plate then being immersed 

 a second time. By a repetition of alternate painting and 

 immersing, all the members of the figure are made to increase 

 in depth by fine gradations from the boundary lines to the 

 central parts. This method, specially applicable to figures 

 with very broad and narrow lines, is, however, rather trouble- 

 some, and does not give satisfactory results unless the grada- 

 tions are minute, which otherwise will be brought out by 

 reflexion on to the screen. 



Reverting to single immersions, final experiments were 

 made with plates bearing figures prepared in the usual way, 

 viz. by removal of wax, a record being kept of the strength 

 of the acid and the time of immersion in each case. The 

 result is that for figures with lines ranging from y 1 ^ to | of 

 an inch in breadth, an immersion of 3 seconds in a solution 

 of 5 volumes of concentrated nitric acid to 2 of water gives 

 a satisfactory depth. 



The next step was to produce figures in low relief, which 

 come out on the screen in shade. As there is in this case no 

 intersection of the reflected rays, and, consequently, no 

 blotting out or dimming of the figures, very narrow lines may 

 be used. These are conveniently drawn with a camel's-hair 

 brush and sealing-wax dissolved in naphtha. From 1 to 2 

 seconds'' immersion, according to the breadth of the characters, 

 will be found sufficient. 



