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are not. From Clark-Maxwell's definition they are purely 

 geometrical abstractions, like " rays of light " in geometrical 

 optics ; and in this sense there is an infinite number of" lines" 

 traversing every field, just as there is an infinite number of 

 " rays " in every beam of light. But we may give a quantitative 

 meaning to both terms by a simple convention : just as we may 

 say that the brightness of the illumination of a plane surface is 

 proportional to the number of " unit rays " that fall vertically on 

 it per unit of area, so we may say that the intensity of a force 

 across any region of a field is proportional to the number of 

 " unit lines " that cut normally the unit of area in that region. 

 This is, indeed, largely the basis of the calculations of the 

 electrical engineer; the C.G.S. "unit line" will exert a dyne 



Fig. 4. — Rhumbler's net, pulled in at two centres ; the direction of the lines of force 

 indicated by thickenings (modified from Rhumbler and Hartog). 



per cm. square ; but of course we might take the microdyne per 

 micron square as the force of our unit line, etc. 



Turning to Gallardo's earlier view, that the mitotic figure 

 represents the lines of force conditioned by a dual or bipolar 

 force centred on the centrosomes, as two opposite poles, we find 

 that this is inconsistent with Faraday's treatment, which only 

 dealt with abstract geometrical conceptions. To render it 

 applicable to the material conditions of the cell, I was obliged 

 to amplify it by introducing from the domain of electricity and 

 magnetism the conception of relative permeability of different 

 substances, with the recognition of an undescribed configuration 

 of matter, material " chains of force " due to redistribution of 

 substances of unequal permeability in a field of force. Thanks 

 to this enlargement, we shall find that Gallardo's explanation 

 no longer presents those gaps which had hitherto invited the 

 attacks of plausible opponents, and that the a priori arguments 



