1 6 INTRODUCTION. 



of the cells of a particular tissue, it is indispensable that 

 the student carefully compare the transverse and longi- 

 tudinal sections of the cells. Moreover the longitudi- 

 nal sections must be compared with the transverse to 

 determine their position. 



It frequently becomes necessary to examine a toler- 

 ably thick object. In such a case, very different views 

 of the object will be obtained as the focusing screw of 

 the fine adjustment is moved. It must be remembered 

 that a good objective gives a clear image of only a 

 single plane at one time, though adjacent images 

 modify this somewhat. Hence it is easy to determine, 

 knowing in which direction the objective is moved by 

 the focusing screw, whether one object is above or 

 below another. 



The use of the fine adjustment must be learned as 

 soon as possible and must be assiduously practiced. 

 The finger should be kept on the fine adjustment most 

 of the time when using high powers, and nothing 

 allowed to escape the vision which the fraction of a 

 turn would reveal. 



VIII. DRAWING. 



In the systematic examination of an object two kinds 

 of memoranda should be made, descriptions and 

 drawings. The value of the former is usually conceded, 

 but that of the latter is often deemed too slight to re- 

 pay the trouble. The importance of drawing can not, 

 however, be too strenuously urged, and the difficulty 

 and tediousness of execution, which will largely dis- 

 appear with practice, should never be offered as an 

 excuse for its neglect. 



