MR. ROY K. MARSHALL continues his articles on this subject in the December and January issues of Sky and Telescope (see NATURE, 153, 191; Feb. 12, 1944) with a very full description of the Fels Planetarium at the Franklin Institute, Philadelphia. He includes a few photographs of other planetaria also. The technical details of the instrument will prove interesting to many readers. The most complicated feature of the planetarium is the mechanism which reproduces the motions of the sun, moon and planets, and it is remarkable that comparatively few visitors make any inquiries about this. Five projectors are in the sun cage, and two of these are for a glow of light or aureole around the sun's image, simulating the strong scattering of light seen in the neighbourhood of the sun. One is for the zodiacal light, and a pair for the glow of the gegenschein. The precession of the equinoxes is reproduced in the planetarium by rotating the dumbbell portion of the instrument about an axis, and in 75 seconds it is possible to pass through the whole cycle of nearly 26,000 years. On certain occasions demonstrations of a highly dramatic nature have been presented—including a trip to the moon during which very realistic reproductions of lunar craters are produced. In the words of Dr. Philip Fox, describing the first Adler Planetarium, it is "not a trivial plaything, a mimic aping firmament, but the heavens portrayed in great dignity and splendour, dynamic, inspiring, in a way that dispels the mystery but retains the majesty".