If the definition of simultaneity is given from a moving system, the spherical surface will result when Einstein's definition with є = 1/2 is used, s… - Hans Reichenbach

If E₁ is the cause of E₂, then a small variation (a mark) in E₁ is associated with a small variation in E₂, whereas small variations in E₂ are not associated with variations in E₁. If we wish to express even more clearly that this concept does not contain the concept of temporal order, we can express it in the following form, where events that show a slight variation are designated E*: E₁E₂, E₁*E₂*, E₁E₂* and never the combination E₁*E₂.

...introduce the auxiliary concept of first-signal...defined as the fastest message carrier between any two points in space. We now send a first-signal from P, calling the event of departure E₁... The event of its arrival at P' is called E'. Simultaneously with the arrival of this signal, another first signal is sent from P'. The arrival of this signal at P is the event E₂. ...the time interval between E₁ and E₂ is coordinated to the event E', [E₁ is earlier than E' and E₂ is later than E'] and every event of this time interval except for the endpoints is inderterminate as to the time order relative to E'.

Visual forms are not perceived differently from colors or brightness. They are sense qualities, and the visual character of geometry consists in these sense qualities.