VISION. 205 



second upright piece having in its center a large hole. 

 Over this hole, on the inner surface of the upright, fix a 

 sheet of lead or of copper in which some figure has been 

 cut (o). Construct a lens carrier (c), whose pointer (p) 

 will indicate upon the scale (s') the position of the center 

 of the lens. The use of the instrument will be some- 

 what facilitated if the distance between the surface of 

 the screen and the surface of the lead or copper be pur- 

 posely made exactly 100 cm. In addition to the above 

 apparatus one needs the lenses whose focal distance he 

 has determined. He needs also a lamp or candle to 

 place behind the metallic screen at e. 



2. Experiments and Observations, Place a light behind the 

 metallic screen; it shines through the figure cut through 

 the screen. This figure is the object. 



(1) (a} Place a lens in the carrier and so adjust it that 

 the plane which it represents is perpendicular to the 

 axis of the instrument and its center is in the same 

 perpendicular plane with the index (p) of the carrier. 

 () Slide the carrier along the base until the object is 



sharply focused upon the screen. 



(c) Read from the scale the distance of the lens from 

 the image (i). If the instrument is made just 100 cm. 

 between screen and object, then the difference be- 

 tween 100 and the reading will be the distance of the 

 lens from the object. Is the image erect or inverted? 

 Explain the phenomenon, drawing geometric figure. 



(2) Study the general formula: 



() S+?4 



() F=^pj; but o+i=rlOO; therefore 

 (^ 100 F = i. 

 From this form of the statement it is evident that the 



