CHAPTBE II. 



OPTICAL CONSTRUCTION. 



Preliminary Note. 



In the former part of this book we have dealt exclusively 

 with the stand, or mechanical means of employing the 

 optical system and accessories ; and important as it is that 

 those details shall be very efficient, it is, if anything, 

 still more so that the eyepieces, objectives, and illumi- 

 nating apparatus shall be of the most perfect description, 

 properly adapted and intelligently employed, for on the 

 optical combinations depend the results that are to be 

 obtained with the stand ; and although care and trouble 

 may enable a person to use a bad stand, no good stand can 

 ever compensate in any way for bad objectives. It requires 

 constant practice and a long apprenticeship to learn to use 

 the microscope to the utmost advantage. Every special 

 subject of examination calls for special manipulative treat- 

 ment if it is to be correctly understood and appreciated. 

 Experience alone can guide in obtaining the best result 

 under varied circumstances, and that experience must be 

 based on a knowledge and understanding of correct methods 

 in working. 



Definitions. 



Some of the following terms will be made use of in this 

 book, and are constantly met with in literature on micro- 

 scopical subjects ; a brief explanation of them may therefore 

 prove of service. 



