192 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



The greater the distance from the object, and the higher 

 the case containing the glasses is from the drawing paper, 

 the larger the image will be represented, which is obtained 

 by the sliding tube, and fixed to any point by the tightening 

 screw ; on the contrary, the nearer the object and shorter 

 the instrument, the smaller the drawing. The sliding tube 

 is divided for the purpose of ascertaining the height of the 

 instrument, should the drawing not be finished at one time. 



Some little attention is necessary to the position for taking 

 profiles, sketching flowers, <fec. Darken one half of the 

 window to shade the instrument, and place the object on 

 the opposite part of the table in the light, having the table 

 close to the window ; the latter always to the right hand, 

 as in the manner usually adopted by artists when taking 

 likenesses. 



The eye-hole should be kept closed when the instrument 

 is not in use. Should the mirror become dull from the 

 damp or dust, remove the bottom by means of the screw. 

 Slide the front glass gently downwards for the purpose of 

 cleaning the mirror. Care being taken, the instrument will 

 be as perfect as ever. Price, $10.00 and $12.00. 



708 - Polemoscope. 



(Fig. 705.) A re- 

 flective perspective 

 glass, invented by 

 Hevelius, who re- 

 commends it as use- 

 ful in sieges, <fec., for 

 discovering what the enemy is doing, while the spectator is 

 hid behind an obstacle. It is, however, more used by 

 lecturers to illustrate the phenomena of reflection. The 

 annexed is a section of the instrument ; it consists of a rect- 

 angular box, two or three inches square, bent four times at 

 right angles, and containing at those angles four looking- 

 glasses, placed at the angle of 135 with the adjacent 

 sides. Suppose a ray of light to enter at A, it would im- 

 pinge upon B, and be there reflected to c. At c a second 

 reflection takes place, which passes the light to D. A third 

 reflection carries it to E, and a fourth reflection sends it to 

 the eye. It is evident the instrument will set equally well 

 when a solid is interposed between the one arm or extremity 

 and the other. Price, $5.00. 



