In a perfectly adjusted instrument, the fundamental lines bear relations (Figure 21.1) as follows:
1. The vertical cross hair should lie in a plane perpendicular to the horizontal axis .
2. The axis of each plate level should lie in a plane perpendicular to the vertical axis
3. The horizontal axis should be perpendicular to the vertical axis .
4. The axis of the telescope level should be parallel to the line of sight
5. The line of sight should be perpendicular to the horizontal axis at its intersection with the vertical axis . Also, the optical axis , the axis of the objective slide , and the line of sight should coincide.