ZOOLOGY AND BOTANY, MICEOSCOPY, ETO. 



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may be used for three purposes : (1) as a means of oblique illumina- 

 tion ; (2) if tlie form of the object be known it will show whether 

 the refractive power is greater or less than that of the surrounding 

 medium ; (3) if the refractive power be known it will show in which 



Fig. 66. 



Fig. 67. 



directions the thickness of the object increases or diminishes. Prof. 

 Exner has also used it to measure the refractive index by immersing 

 the object in different liquids whose refractive index is known, and 

 so finding two of very nearly 

 the same refractive power be- 

 tween which it must lie. He 

 considers that the method is 

 so sensitive as to measure the 

 index accurately to a few 

 imits in the fourth decimal 

 place. 



As an application of the 

 method it is shown how the 

 optical constants of the eye of 



Hydrophilus piceiis were determined. An examination of the cornea 

 proved that for each facet the index of refraction increases towards its 

 centre so that the facets may be regarded as consisting of a number 

 of cylinders inclosed within one another, the innermost having the 

 strongest refractive power. This is just such a structure as will con- 

 centrate the greatest possible number of rays which fall upon each 

 facet towards its axis, so that they may be utilized in the act of vision. 



