5. Optical Notes

Extract

The second figure in Professor Everett's note (ante, p. 360) has reminded me of my explanation of a phenomenon which I have repeatedly seen for more than twenty years in the College. When sunlight enters my apparatus-room through a vertical chink between the edge of the blind and the window-frame, the line of light formed on the wall or floor shows a well-marked kink. Similar phenomena, though not usually so well marked, are often seen in old houses, when the sun shines through the chinks of a Venetian blind. They are obviously due to inequalities (bull's-eyes) in the glass which was used more than a generation ago for window-panes. Professor Everett's figure, which was drawn for a cylindrical lens, represents the general form of a central section of such a bull's-eye.