STARCHES UNDER POLARISED LIGHT. 



243 



immediately above the objective. This plate is usually 

 3 '75 millimetres in thickness, and is of the greatest utility 

 when very feeble doubly refracting crystals are being 

 examined, and in many other instances where it is sus- 

 pected that isotropic particles exist in admixture with 

 amorphous matter of a doubtful character. 



The action of two Nicol prisms with an interposed plate 

 of quartz upon a beam of light has been made use of for 

 the purpose of registering the position of absorption bands 

 in the spectra of various substances ; but this properly 

 belongs to the next chapter. 



The development of the study of objects by the aid of 

 polarised light may in the end bring about important dis- 

 coveries, and therefore should be studied more carefully 

 than it is at present. 



Let the student examine some potato starch by the use 

 of ordinary light, and after noticing the position of the 

 hilum, again examine 

 with polarised light, 

 when he will not fail 

 to notice the black 

 cross, as shown in 

 Fig. 213. 



For the observation 

 of starches under po- 

 larised light, they 

 should be mounted 

 in gum dammar dis- 

 solved in benzol as 

 in aqueous fluids the 

 effect is not developed 

 in so marked a manner. Wheat starch, which only faintly 

 shows the cross when observed in water, exhibits it very 

 plainly in gum dammar and benzol. The effects of polarised 



R 2 



FIG. 213. 



