18 SEC. 2. GEOMETRY. 



67. Models of Mathematical Instruments. The ortho- 

 compass and the addition compass. Prof. L. Zmurko, Lemberg. 



The first of these instruments is constructed so that the points of the compass 

 are always parallel to each other, and perpendicular to the surface of the 

 paper. The second is a compass which can be used also as a protractor, as it 

 contains an apparatus which indicates the amount of opening between the 

 arms. 



69a. Photographs of Mathematical Instruments. 



Otto Fennel, Cassel. 



69d. Revoil Tele-iconograph, altered for perspective draw- 

 ings enlarged to 20 times on a horizontal plane-table. 



M. Georges Sarasin, Geneva. 



The instrument consists of a telescope, adapted to a Wollaston camera lucida, 

 and fixed on a stand arranged so as to make it a mathematical or scientific 

 instrument ; while on a separate stand is placed a plane table for drawing. In 

 order to facilitate the exact grouping of the partial perspectives in accordance 

 with a general cylindrical perspective, and capable of being developed, and in 

 order to permit of drawing while the telescope is inclined at great angles, 

 the following additions have been made to the Revoil model: 1st. A 

 tightening ring with an adjusting screw, which fixes the prism to any point 

 on the thread of the screw by which it is fixed to the eye piece. 2d. A web 

 of six threads crossed at right angles in the focus of the object glass for 

 the purpose of setting the partial images in a straight line and in a direction 

 in accordance with the horizontal or vertical. 3d. A spirit level on the 

 telescope stand to ensure the verticality of the axis of rotation. 4th. A 

 socket and rack joint, permitting the height of the prism above the drawing 

 to be determined whatever be the angle of the telescope and, consequently, 

 the scale of the drawing. 5th. A graduated scale with vernier, giving a 

 reading to five minutes on the horizontal limb. 6th. A method of attaching 

 the instrument to its stand, so as to be at the same time firm and easy to 

 work. 



69e. Patent Dotting Pen. 



E. O. Richter and Co., Chemnitz, Saxony. 



The arrangement is as follows : 



A small toothed wheel adapted to the kind of dotted lines to be drawn is 

 attached to a plate, which, rolling on the paper, lifts a lever, which is again 

 dropped by means of a spring. Attached to this arm is a drawing-pen, easily 

 adjustable, by means of a hinge, to the correct position suited to the wheel. 

 The wheel itself is held in position by a small, somewhat elastic plate, which 

 can be displaced a little for fitting the different reserve wheels on which 

 the kind of lines to be drawn depends. 



69f. Patent Compasses with stationary centre point. 



E. O. Richter and Co., Chemnitz, Saxony. 



These compasses differ from others, chiefly by the centre-point being 

 stationary on the paper, so that the tracing-pen, resting on the paper by its own 

 gravity, moves about the centre-point as axis, whilst the movable pen can be 

 displaced, without removing the compasses from the paper. By these advan- 

 tages speedy and neat tracing will be achieved, even when the smallest circles 

 are to be drawn. 



