6/31 The rod should be at right-angles to the line of sight to its centre. Fasten another rod at right angles to the first. From the point at which the rods cross measure off and mark on both rods spaces each subtending a degree as seen from the point of view. Thus, if the point of view is 9-1/2 feet off, these spaces must each be 2 inches long, and they must be proportionately less or greater as the eye is nearer or farther. 10._] Now suppose the observer wishes to view Mercury on some day, whereon Mercury is an evening star. |