Sentence:  Yet it is not until Book X that the properties of such a line (with greater length is an <WSD>apotome</WSD> and lesser length a first <WSD>apotome</WSD>) are explained and not until Book XIII that this type of line is applied, which will be discussed in more detail later....The likewise is true of apotomes (X. 97). 
Meaning of the word marked with <WSD> tags:  The difference between two quantities or lengths commensurable only in power, as between 1 and the square root of 2, or between the diagonal and side of a square.