INTRODUCTION. 5 



done with carefully directed and modified light and 

 an equally careful arrangement of magnifying 

 power, yet the utility of the judicious application 

 of re-agents requires only to be understood to be 

 appreciated. The knowledge of the chemical con- 

 stitution, physical properties and modes of manu- 

 facture of re-agents, often leads to the discovery 

 of their specific action on minute and ultimate 

 structure, whether these may consist in a revelation 

 of inherent qualities or in a modification of details. 

 It thus becomes incumbent upon the histologist 

 to most carefully scrutinise the action of every 

 description of re-agent, and equally carefully and 

 systematically to record the results of such ex- 

 aminations. 



Re-agents may be used in a variety of ways for 

 the elucidation of structure and the detection of 

 the constitution of tissues and bodies. Some are 

 employed as chemical tests, others as staining 

 media to differentiate the tissues or to induce 

 chaoges and appearances whereby certain proper- 

 ties are revealed, whilst others, again, are used as 

 preservative media. To know exactly what to 

 apply and how to apply it for the revelation of 

 specific phenomena, and to obtain the best results 

 is of the very essence of microscopical technique 

 and manipulation. 



III. On the Methods of Preparation. 



In order to arrive at a correct knowledge of, and 

 as a means to their study, both organic and in- 

 organic specimens and the great majority of or- 

 ganisms require preparation by special methods. 



