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bourne.* Now Minkowski has used the fourth dimension of the 
nature of time as being of the same essential character as the 
others, and so while what we happen to regard as purely height- 
in Perth may be regarded as purely breadth in Cape Colony, it 
might be regarded as of the nature of time in some other world 
possessing a velocity different from that of our Earth, The four- 
dimensional construction is very convenient as connecting together 
what we term position (or space) and time, so that a graph in it 
gives the whole history of progress of a particle in our universe, 
Avhat we term the “world-line" of the particle. This four- 
dimensional world is spoken of as Minkowski’s space-time world, 
and we gather from it that it is impossible to obtain an absolute 
separation into space and into time, but only a relative separation 
made to suit the particular observer. In Minkowski’s own words, 
‘‘Henceforth space and time in themselves vanish into shadows, 
and only a kind of union of the two preserves an independent ex- 
istence.” This idea is referred to as the principle of relativity, 
and we picture the aether as a four-dimensional continuum filling 
uniformly Minkowski’s space-time world. In short the position is 
this, that just as we have regarded such properties as the colour 
and scent of a rose as dependent on the acuteness and accuracy 
of the observer's, senses of sight and smell, so we must regard all 
ideas of form, position and time as purely relative and as varying 
for observers on different worlds having different motions relative 
to the aether. Time is no more absolute than our ideas of taste, 
touch, smell, colour and sound. 
Ail observation consists in the recording of coincidences. For 
example, in measuring the size of a microscopic object we note the 
coincidence of the ends of the object with two lines on two scale 
divisions in a micrometer eyepiece. Hence as the world-line of a 
particle gives its full history, observations are merely the dis- 
covery of intersections of these woi ld-lines, and we know of the 
action of a force on a particle by the deflection produced in the 
world-line of the particle. When there is no external action, the 
world-line runs straight. The gravitational influence of a particle 
throughout its neighbourhood, which leads to it affecting other 
particles and deviating their world- lilies, has been accounted for 
on a theory which, while it in no way explains the cause of gravita- 
tion, brings that action for the first time under the same rationale 
as other forces. It is assumed that the gravitational held sur- 
rounding a particle is equivalent to a strain or distortion of that 
portion of Minkowski’s space-time world, and that the orbit due to 
gravitational action of a second particle about the first is a path 
* In mathematical language, a. vector which is parallel to the vertical axis OZ for 
Perth, has ooinpommt'-i along the vertical axis OZ and the horizontal axis OY at 
Sydney, and components along the axes OZ and OX for Roehourne. 
