3 6o 



OPTICAL PROJECTION 



Shrimp and prawn shells, and the larger fish scales, make 

 good objects. With the polarising microscope, all the usual 

 organic preparations are available, and starch can be fairly 

 shown even with the oxy-hydrogen light, and excellently with 

 the arc. 



209. Strains. The effects of strain or tension are most 

 easily shown in glass. The usual wooden press frames sold 

 for the purpose exhibit the phenomena distinctly enough, but 

 the pressure is insufficient to do so with the beauty they 

 are capable of. I therefore constructed a frame of solid 



gun-metal, with a 

 screw turned by 

 a powerful T-key, 

 as in fig. 201, the 

 glass being com- 

 pressed between 

 the convex sur- 

 faces of c and A. 

 To avoid any 

 twisting strain on 

 the polariscope- 

 stage, it is better 

 to have a screw 

 at both ends of 

 the slide, and to 

 use two keys, when one twist balances the other, and more 

 pressure can be put on. The glass should be compressed 

 even to breaking, if possible ; for the most brilliant chromatic 

 fringes are produced under the greatest strain. Before 

 breakage, however, the stage with the press in it should be 

 rotated 45, in order to exhibit the great difference in the 

 optical results in that position. 



The same press, arranged as in fig. 202, with a narrower 

 piece of glass resting against two pieces of brass B B as abut- 

 ments, c pressing between them as before, gives the optical 



FIG. 201 



FIG. 202 



