36 SEC. 2. GEOMETRY. 



128. Drawings. A collection, executed by the Students of 

 the Bohemian Polytechnic Institute, illustrative of the instruction 

 received in the subject of Organic Geometry of Form. 



Prof. Franz Tilscr, Prague. 



129. Two specimens of Wire Stereometrical Models, 



with letters on cork. 



Prof. J. Joseph Oppcl, Frankfort-on- Maine. 



ISO. Two specimens of Wire Trigonometrical Models, 



with letters. Prof. J. Joseph Oppel, Frankfort-on- Maine. 



131. Two specimens of Wooden Stereometrical Mo- 

 dels, with letters. 



Prof. J. Joseph Oppel, Frankfort-on- Maine. 



The auxiliary lines, diagonals, &c. are distinguished by wires of different 

 colours or thicknesses. They are in many cases movable, so that the perfect 

 figure can be constructed before the eyes of the pupil. 



Auxiliary planes are also distinguished by their colour. The angular 

 points are provided with metal pins, to which letters on cork discs can be 

 attached, so as to be turned upright towards the observer. 



These models have proved highly serviceable for instruction during the 

 past 20 years. 



132. Large Model of an Ellipsoid, of white cardboard, 

 on a turned stand. Prof. Dr. A. Brill, Munich. 



133. Cardboard Models of Surfaces of the second 

 order, on frames. Made up of circular sections. The sections 

 are attached to each other. Prof' Dr. A. Brill, Munich. 



This collection of models consists of : 



1. Ellipsoid having 20 circular sections. 



2. Ellipsoid having 30 circular sections. 



3. Hyperboloid of one sheet. 



4. Hyperboloid of two sheets. 



5. Elliptic Paraboloid. 



6. Cone in two sheets. 



7. Hyperbolic Paraboloid. 



141. Series of Cardboard Models of Surfaces, of the 

 second order, in a cardboard box. The sections are not 

 attached to each other. Prof. Dr. A. Brill, Munich. 



These models, Nos. 132, 133, 141, are distinguished from those in common 

 use by their mobility, by means of which each one represents not only a single 

 ellipsoid or hyperboloid, but a series of surfaces of one or the other kind. 

 For when the angle of inclination of the circular sections is altered, in a direc- 

 tion easily recognised by pressing or drawing out the model, there will be 

 obtained a simple but infinite system, the individual forms of which can be 

 converted from a flat figure through gradually-changing solid bodies to just 

 such another figure with a different relation of axes, without, however, losing 

 its properties. 



