36 THE ART OF PROJECTING. 



of a vibrating cord, or large tuning-fork ; or the ap- 

 paratus for showing the linear expansion of metallic 

 rods, etc. The following method will be found applica- 

 ble to a great many such cases, where simply the outline 

 of the instrument is needed. 



Place a short focus objective (and the shorter the 

 better), so near the focus of the condenser that all the 

 light falls upon it. After refraction the light will form 

 a very divergent beam and the focus in front of o will 



Fig. 21. 



be a sharp point, practically a luminous point, and any 

 object held between it and the screen s, will have a 

 strong shadow cast upon the latter. The magnitude of 

 this shadow will depend upon the distance from the 

 focus. There will be no penumbra the outline will be 

 sharply defined. 



If one has a lantern, the condensing lens above will 

 answer without the objective, as its focus for parallel 

 rays will be sufficiently short. A globular glass flask, 

 filled with water and placed m the path of the rays, will 

 also be found to be satisfactory. When a lantern is 

 used instead of sunlight, it will be necessary to use the 

 microscope attachment, which is described further on, 



