5-2 



KNOWLEDGE. 



[March 1, 1900. 



size is sometimes far greater than this. It is unnecessary 

 to rcjiroduce the positive figure, as it does not differ in 

 such a marked degree from the iJOsitive figures obtained 

 on plates. The substitution of films for plates does not 

 make much difference in the images got in respect 



FiO. 3. — JTornml negative on paper. 



either of their forms or the coiTespondence in size o( 

 the positives and negatives. 



The smaller size of Fig. 3, as compared with the 

 companion positive figure, appears to be owing, in part, 

 at all events, to the greater penetrative power of the 

 discharge proceeding from the negative terminal, which 

 carries a portion of the electricity through the gelatine, 

 and into and beyond the pa2)er support, so that only 

 a part of it produces visible effects- by acting on the 

 silver in the sensitive emulsion. This may be proved 

 by placing two sensitized jiapers (b and c) on each side 

 of a piece of glass, a film of gelatine over each, and 

 above the gelatine films two other sensitized papers 

 (a and d), and insei-ting the arrangement between the 

 terminals of the discharger, a and b being on the side 

 next the positive terminal. If a single discharge be 



7 



Fig. 4. — Negative on Nikko paper with Leydeii jar iu circuit. 



caused to pass between the temiinals, the negativs 

 electricity will pass through d, and its underlying 

 gelatine to c to a greater extent than the positive does 



through A and its film to b, and will thus produce a 

 larger figure on c than that on b, while the negative 

 figure on d will, for the same reason, be smaller than 

 the positive one on a. In several cases I found tha 

 paper b a complete blank, while c showed a well-marked 

 image. 



It would also ajjpear that the form of the image repre- 

 sented by Fig. 2, as well as the smaller size of the 

 negative images on paper, is dependent on this more 

 effective action of the negative side of the discharge. 

 In the first place, the negative fronds do not aijpear, 

 even on plates, unless the action of the discharge is 

 strengthened by induction, as has been shown by the 

 experiments both of Mr. Brown and Lord Armstrong ; 

 and even if the inductive action be preserved, but the 

 strength of the current be reduced, the resulting figures 

 will not only be smaller in size, but be wanting in the 

 characteristic branching character of the positive and 

 the fronds of the negative. 



On the other hand, the normal form of the negative 

 figure on plates may be more or less closely reproduced 

 on sufficiently rapid sensitized papers if the discharge 

 on them be condensed by connecting a Leyden jar with 

 the t«nninals of the coil, so as to act as a shunt. The 

 figures thus obtained show, in addition to the fronds, 

 tlie ordinary rayed paper form of the negative discharge 

 (see Fig. 4), which appears to be produced by a poi'tion 

 of the cun-ent which passes directly to the paper without 



Fig 5 ^Exterior paper negative witli Levden jar. 



jjroceeding along the shunt wires that lead to the jar. 

 If two pieces of paper be used, one above the other, as 

 in the quadruple sets just described, this direct current 

 will be too feeble to penetrate the outer paper d, while 

 the other portion of the cuiTent, condensed by the 

 Leyden jar, will pass through D, and act solely on c. 

 D will thus show only the ordinary paper figure (Fig. 5), 

 and c will show one composed entirely of fronds (Fig. G). 

 The fronds can, however, be produced on paper witii 

 greater ease and certainty by other methods. If oil be 

 Sf)read on the back of sufficiently rapid paper, so as to 

 make it translucent, before it is exposed to the discharge, 

 the negative image will then be covered with fronds 

 as handsome as those produced on glass, and it will also 

 be of equal size with the positive image, which, for its 

 part, will contain the branchings chai'acteristic of posi- 

 tives on glass plates. The same results may be obtained 

 in a smaller degree by tiu-ning the sensitized side of 

 the paper which is neai- the negative terminal towards 

 the glass, or by keeping the sensitized side outwards, 

 and covering it with a thin sheet of glass, gelatine, or 

 the like. In the last mentioned case the images are 

 somewhat bluiTed, and the fronds sometimes very small, 



