184 SPECIAL PHYSIOLOGY 



fix a sheet of lead or of copper in which some figure has been cut. 

 Construct a lens carrier (c), whose pointer (p) will indicate upon 

 the scale (V) the position of the centre of the lens. The use of the 

 instrument will be somewhat 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 is so determine. He 

 needs also a lamp or candle to place behind the metallic screen at e. 



FIG. 77 



V 



Apparatus for determining the principal focal distance through the observation of the con- 

 Jugate focal distances : o, object ; I, lens ; i, image (the conjugate focal distances o I and i I may 

 be represented by o and i, respectively) ; c, lens carrier, which slides along the guide on the 

 bottom of the box. 



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 (o). 



(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. 



(6) 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 the screen and 

 object, then the difference between 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. 



(&) F - ; but o + i = 100 ; therefore 



(c) 100 F = o i 



(<0 ... F= ' 



100 



From this form of the statement it is evident that the lens will 

 throw a distinct image in either one of two positions. Demonstrate 

 it experimentally. 



(3) Determine o and i for each lens and substituting their values 

 in the equation (d) determine the value of F. A slight deviation 



