The arrows have been retained.¹

1. The direction of an arrow should not be taken as a direction to the reader.
2. The figure reproduced here includes a small observer at the bottom. No name is given.
3. The circular outline is incomplete near the observer. This is visible in the supplied image, not a printing error on this page.
4. The position of the observer determines what is meant by left and right. See also the correction sheet.
5. No scale has been supplied.
6. The main text to which these notes refer is not included.

find the missing context