Excerpt from The Rotation Period of the Sun: As Determined From the Motions of the Calcium Flocculi In measuring photographs distorted in this way the necessary correction for a point at a given distance from the Sun's limb might be taken from a table, readily constructed for a given position of the Sun's image with respect to the axis of the collimator. To define this position, means were provided for making the solar image concentric with the axis of the collimator. Care was always taken to orient the image so that the distorted axis should be parallel to the solar equator in the photograph. For this purpose the whole instrument could be rotated about the axis of the collimator, the direction of the slit being read 03 on a position circle. The parallel lines on the photo graph (due to dust on the slit, which can not be altogether avoided in any form of spectroheliograph when the slit is narrow) were made to serve a useful purpose in the orientation of the image. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.