TEACHING AND LEARNING 19 



The student will have learned in his earlier courses the char- 

 acteristics of good scientific drawing, that exact faithfulness 

 to fact, that diagrammatic clearness, that complete accuracy 

 in detail, which are so far from the impressionist effects sought 

 by artists. In this course he will find that outline diagrammatic 

 drawings will be of most use in illustration, especially of appara- 

 tus; and he should learn, through study of good models, to make 

 them. Such drawings are of two kinds. First, there are the 

 mechanical sections, used extensively in this book; they are 

 easy to draw by aid of the simple instruments of mechanical 

 drawing, and are valuable in showing the exact construction of 

 apparatus. Second, there are external outline views, illustrated 

 by figure 40 in this book; they are decidedly more pleasing than 

 sections to the eye, but, involving perspective, are more difficult 

 to draw, and they are also somewhat less illustrative of details 

 of construction. These the student should learn to draw free- 

 hand, but he should also know how to make use of the semi- 

 mechanical method of outlining in waterproof ink, and then 

 bleaching, a photographic blue-print, details for the accom- 

 plishment of which will be found under Manipulation in Part 

 III. He should learn also something of the modes of repro- 

 ducing illustrations for publication, upon which there is a good 

 article in Encyclopaedia Britannica, 32, 13, and of the methods 

 of preparing drawings for such reproduction, upon which there 

 is a very valuable article by BARNES in the Botanical Gazette, 



43, i97> 59- 



An invaluable kind of illustration is the graph, by which 



statistical data are recorded in curves or polygons. As illus- 

 trations graphs bear very much the same relations to tables of 

 figures that pictures do to pages of words, that is, they not only 

 express results clearly to the eye at a glance, but they also 

 bring out facts and relations which would not be discovered 

 by even a minute inspection of words and figures. Though 

 valuable for representation of all quantitative relationships, 

 such graphs are especially illuminating when two or more sets 

 of data are to be compared. They are plotted on co-ordinate 



