THE METHODS 



OF 



MICROSCOPICAL EESEABCH, 



INTRODUCTION. 



I. On Instruments and their Use. 



THE investigation of minute structure, whether 

 organic or inorganic, can be exhaustively and satis- 

 factorily accomplished only by the assistance of 

 adequate optical instruments. The organ of vision 

 in man is so constructed that the distance of most 

 distinct vision alternates between eight to ten 

 inches, and the intermediates between these dis- 

 tances, from the normal eye of an adult. This is 

 due to the alterable curvature of the crystalline lens 

 which allows the practically parallel pencil of rays 

 from a distant object, or the divergent rays from an 

 object close to the eye, to be accurately focussed on 

 the retina, or sensitive portion of that organ ; but if 

 the object is brought still closer to the eye than the 

 normal distance of most distinct vision, it gradually 

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