SPACE 29 



with two object-glasses of equal foeal lengths. The 

 mountings of these objectives must be cleft by a deep 

 slot, perpendicular to their axes ; and within this slot 

 there must rotate a diaphragm perforated by two 

 openings, which are diametrically opposite to one 

 another. This diaphragm rotates at a uniform rate, 

 and simultaneously uncovers the two objectives; during 

 each complete revolution of the diaphragm the 

 objectives are twice uncovered. A mechanical motor 

 turns the circular diaphragm, and, by means of a pulley 



Fig. 22.— Sphere engendered by tie rotation of a semi-annular band, white o". the 

 outer surface and black on the inner s:de. 



and continuous band, the same motor serves to turn 

 the axis and the white threads. The latter, by their 

 movements, describe the desired figures. 



Figures formed by the Movement of Solid Bodies; 

 Effects of Light and Shade. — Instead of the fine thread, 

 which has just served our purpose for describing the 

 surface of a sphere in space, let us take a solid body ; 

 the appearance of the figure described will be quite 

 different. A band of Bristol board is arched length- 

 wise, its convex surface blackened, and its concave 

 4 



