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In the second place, the co-ordinate axes are supposed to be normal to one another. How is this normality to be defined ? It is to be remembered that normality in Space-Time theory is a wider conception than normality in ordinary geometry ; since in the former we have lines which are " selfnormal" (or what I have called "optical lines"),doi:10.1038/110810a0 fatcat:7iifhj5z4rdevnirhuczn2ubaa