OPTICAL INSTRUMENTS. 



191 



plate of glass in the 

 front, and which is ca- 

 pable of adjustment. 

 The rays from the ob- 

 ject pass through the 

 glass to the reflector, 

 and upwards to the 

 eye, which sees them 

 by looking through 

 the small hole at the 

 top. The form or ge- 

 neral arrangement of 

 all these instruments is 

 similar, and easily un- 

 derstood from the cut. 

 The management of 

 the instrument is best 

 given in the inventor's 

 own words, especially 

 as they are applicable 

 to all similar contri- 

 vances. The instru- 

 ment being fixed by 

 the clamp and screw 

 to the table and paper 

 on which the drawing 

 is to be made, look 

 through the eye-hole 

 having the front of the 

 case which contains 

 the glasses opposite 

 the object to be co- 

 pied, adjusting it by means of the joints, and getting the first 

 line perpendicular, the whole of the tracing will prove to be 

 in true perspective. 



If objects can be seen distinctly on the upper part of the 

 paper, and not on the lower, incline the part of the case re- 

 flected downwards by the joints, until the reflected image is 

 on the part of the paper required. Many persons, upon first 

 attempting to use the instrument, occasionally lose sight of 

 their object or pencil by an unintentional motion of the eye ; 

 to avoid which, contract the eye-hole by means of the eye- 

 piece which covers it. 



