PHYSICAL EXPERIMENTS. 51 



has a short focus, and will not be more than three- 

 fourths of an inch in diameter. A strip of clear glass 

 an inch wide and three or four inches long, will answer 

 upon which to spread the solutions to be examined, a 

 few of which are given in another place. The glass 

 will then only need to be placed in its receptacle, and 

 its front focused, the same as for any microscopic 

 objects. Any further instructions that may be needed, 

 may be found under the descriptions of the method 

 with the solar microscope. With the porte lumiere^ and 

 two lenses of proper focal length, the finest effects can 

 be shown. 



Fig. 3'4. 



Let c be the condenser, with say twelve-iuch focus, 

 the objective with one-inch focus ; it may be a common 

 pocket lens, or a linen prover, or a botanical glass. 

 First adjust c so as to give a disk of light upon the 

 screen. The rays will cross at the focus, and diverge 

 afterward. Place the lens so that all the light may 

 pass through it, or as much as possible ; this will de- 

 pend upon the size of 0. At any rate, it will be near 

 the focus of c. Have ready a slip of glass three or 

 four inches long and an inch wide, and wet one side 

 with the solution to» be crystallized ; as, for instance, 

 ammonium chloride, sometimes called sal ammoniac. 

 Place it back of the objective at g, and move it until 



