LENSES. 21 



enter that side or emerge from it, when the source 

 of light is at b. It must not be inferred that noth- 

 ing can be done except with lenses of a particular 

 sort. On the contrary very much can be done with 

 such poor lenses as are used in dark lanterns, and 

 are full of striae and air bubbles. The lenses that 

 come with ordinary magic lanterns will answer for many 

 purposes. Spectacle glasses, linen provers, botanical 

 glasses, are all very useful. A pow- 

 erful lens can be made out of two 

 watch glasses, one (a) a little larger 

 than the other (b). Bring the two 

 together under clear water. When 

 raised out of the water they will 

 adhere quite strongly, and for a time 

 can be used to advantage as a mag- 

 nifier. 



FOCAL LENGTH. 

 Fig. 13. The focal length of a lens should 



be known before it is brought into use, and it may be 

 determined experimentally in the following way : 



With the lens placed as shown in Fig. 12, so that the 

 parallel rays from the porte lumiere fall squarely upon 

 it, measure the distance from the centre of the lens to 

 the point b, the focus. If the lens be double-convex, 

 add one-half its thickness to the measured line. This 

 number will represent the focal length of the lens. If 

 the lens be plano-convex, its focal length will be the 

 distance from its flat side (Fig.i2) to the focus. Again, 

 hold the lens so that the direct rays of the sun fall per- 

 pendicularly upon it, and measure as before the dis- 

 tance to the focus. Lastly, if the sun be not shining, 

 bring the lens close to a white wall or sheet of paper 

 opposite to a window, and hold it so that the light from 



