204 



LABORATORY GUIDE IN PHYSIOLOGY. 



say: The reciprocal of the distance of the object from the lens 

 ( ) plus the reciprocal of the distance of the image (1) 

 equals the reciprocal of the general focal distance ^ : thus 



(-i--[~-f -p ) This formula enables one to compute 

 the focal distance after first determining by experiment the 

 values o and i. Inasmuch as the student has already deter- 

 mined the focal distance (F) and may not have made the 

 rather extended computation incident to the derivation of 

 the above most valuable formula it is considered that the 

 most profitable course to pursue at this point is the verifi- 

 cation of the formula. 



rprrT 



FIG. 28. 



FIG. 28. An apparatus for determining the conjugate focal distance 

 For description, see c=l. 



/. Apparatus. To that end one may construct a simple 

 apparatus (Fig. 28). For the determination of the focal 

 distance it is usual to have both object and lens mova- 

 ble. For our purpose this may be dispensed with as it 

 lends little to the reliability of the result and detracts 

 much from the simplicity of the apparatus. Upon a 

 thin board as a base fix an upright piece near one end 

 of the base, whose inner surface may be painted white 

 and serve as a screen (S). Near the other end fix a 



