528 



NATURE 



\Sept. 12, 1878 



disc, (5) paraboloidal points. The results of each series 

 are clearly set forth in the tables and plates which accom- 

 pany the original memoir ; here we have only space for 



inch 



0.64 

 0.63 

 0.62 

 0.51 

 O.60 

 0.59 

 0.58 

 0.57 

 0.56 

 0.55 

 0.54 

 0.53 

 0.52 

 0.51 

 O.50 

 0.4-9 

 0.4S 

 0.47 

 0.46 

 0.45 

 0.44 

 0.43 

 0.42 

 0.41 

 O.40 

 0.3S 

 0.38 

 0.37 

 0.3e 

 0.35 

 0.34 

 0.33 

 0.32 

 0.31 



0.3 



0.?9 

 0.28 

 0.27 

 0.26 

 0.25 

 0.24 

 0.23 

 0.22 

 Q.2I 

 0.2c 

 0.1 S 



0.1 e 



0.17 



c.ie 



0.15 

 0.14 

 0.1 3 

 0.12 

 0,1 I 

 0.1 c 

 O.09 

 O.OS 

 0.07 

 O.05 

 O.OS 

 0.04 

 0.03 

 0.02 



0.0 r 



a diagram (Fig. 7) giving a comparative view of the mean 

 curves of all the measurements. 

 An inspection of the diagram will show that with plane 



CELLS 1000 



2000 



5500 6000 7000 <^i^^0 



Fig. 7.— Curves of Length of Spark.. 



inch 



0.64 

 0.63 

 0.62 

 0.61 

 0.60 

 0.59 

 0.58 

 0.57 

 0.58 

 0.55 

 0.54 

 0.53 

 0.52 

 Q.SI 

 0.50 

 0.49 

 0.48 

 0.47 

 0.46 

 0.45 

 0.44 

 0.43 

 0.42 

 0.41 

 0.40 

 0.39 

 0.38 

 0.37 

 0.36 

 0.35 

 0.34 

 0.33 

 0.32 

 0,31 

 0.30 

 0.29 

 0.23 

 0.27 

 0.20 

 0.25 

 0.24 

 0.23 

 0.22 

 0.21 

 0.20 

 0.19 

 0.18 

 0.17 

 0.16 

 0.15 

 0.14 

 0.13 

 0.1 2 

 0.1 1 



b.io 



0.09 

 0.03 

 0.07 

 0.06 

 0.05 

 O.OJ- 

 0.0 J 

 0.02 

 0.01 



or slightly convex surfaces the potentials necessary to 

 produce discharge at various distances are not propor- 

 tional to the distances when these are small (less tlian 

 0*025 inch), but tend to become so as the distances are 



increased. These results confirm and extend those pub- 

 lished by Sir W. Thomson.^ 



{To be continued.') 



PHYSICS IN PHOTOGRAPHY'' 

 II. 

 VVTE have hitherto treated the question of a sensitive 

 * * compound from what may be called a chemist' s 

 point of view, but it has also its physical aspect, and to 

 enable us to understand what has recently been done in 

 photography this latler must be briefly touched upon. 

 To commence with, we are met with a difficulty in no- 

 menclature which ought not to exist. Unhappily chemists 

 and physicists employ the term molecule in a different 

 sense. The physicist's molecule, for instance, in one 



' Continued from p. 491. 



place is defined ^ as " a small mass of matter the parts of 

 which do not part company during the excursions which 

 it makes when the body to which it belongs is hot." To 

 avoid misapprehension the expression molecular group 

 will be used for the physicist's molecule for want of a 

 better, the word particle being rather too indefinite, and 

 being usually applied to a group of molecules of visible 

 size, a state of aggregation which is by no means neces- 

 sary. The question as to the possible variation of the 

 number of molecules composing a molecular group has 

 not been entered into, as it would be trenching on ground 



' Proc. Roy. Soc, vol. x. pp. 326-33S . • , 



* "Theory of Heat," Maxwell. 



