z^ 



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 j, 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 in 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. 



