202 



LAB OR A TOR Y G UIDE IN PH YSIOL O G V. 



diameter. The bottom of the box is constructed as fol- 

 lows: (See Fig. 27.) Cut through the middle of the bot 

 torn a slot about 0.5 cm. wide and 45 cm. long. Make a 

 lens carrier of wood as indicated in the figure (Fig. 27, 

 C. & C'.). Tho saw groove in the top of the carrier 

 serves to hold the lens. If, however, the lenses to be 

 used in the apparatus be not provided with rims and 



FIG. 27. 



FIG. 27. Showing parts of apparatus for determining the focal distance 

 of lenses. For construction of the apparatus, see XLVI=b=2. 



rings the demonstrator can readily contrive a means of 

 holding them in place. In any case they should be so 

 held that the plane of the lens is perpendicular to the 

 axis of the box, and that the center of the lens (o) 

 is virtually over a fixed line (o') drawn transverse to the 



