182 Mr. W. B. Croft on Breath Figures. 



The plan which gives the surest and most beautiful results 

 is to place five or six coins, lying in contact side by side in a 

 cross or star, on either side of the glass : it is not necessary 

 that each coin should exactly face one on the other side. 



There has not appeared airy distinction between the figures 

 made by positive and negative electricity. 



When several coins are side by side, touching one another, 

 there appear in the spaces between them, which are mostly 

 black, well-defined white lines, common tangents to the cir- 

 cular edges of the coins. If these are of equal size the lines 

 are straight ; otherwise they are curved, concave towards a 

 smaller coin. They seem to be traces in that plane of the loci 

 of intersection of equipotential surfaces. 



Similar effects are obtained when coins and glasses are piled 

 up alternately, and the outer coins are put to the poles of the 

 machine. With six glasses and seven coins perfect images 

 have been formed on both sides of each glass. With eight 

 glasses the figures were imperfect ; but there is little doubt 

 this could be improved by continued trials as to the amount 

 of electricity applied. 



If several glasses are superposed and coins are applied to 

 the outer surfaces, there are only the two images at the outside. 

 After the electrification there is a strong cohesion between 

 the plates. 



It requires some practice to manage the electrification so 

 as to produce the best results, There are two forms of failure 

 which present interesting features. Sometimes a picture 

 comes out with the outlines dotted instead of being continuous. 

 At other times, if the electrification is carried too far, the 

 impression comes out wholly black ; but on rubbing the glass 

 when dry with a leather the excess is somehow removed. 

 Naturally it is difficult to rub down exactly to the right point, 

 but I have succeeded on several occasions in developing from 

 a blank all the fine detail of elaborate coins. 



Here, again, we have another instance of the development 

 by lapse of time, for an over-excited piece of glass usually 

 gives a clear picture after an interval of a day or two. 



Impressions from stereotype plates have been taken of 

 which the greater part is legible : the distinctness usually 

 improves alter a few days. In default of a second plate, a 

 piece of tin-foil about the same size should be put on the 

 opposite side of the glass. 



Sheet and plate glass of various thicknesses have been used 

 without any noticeable change either in the treatment or the 

 results. 



I have put an impressed glass on a photographic plate in 



