38 
JOURNAL OF ECONOMIC ENTOMOLOGY 
[Vol. 11 
this the writer suggests that a cardboard model be made by using the 
thickness of the cardboard to represent the contour interval. The 
outlines of the contours can be secured by placing carbon paper on the 
cardboard and underneath the map when the lines can be traced. The 
cardboard can then be cut with a sharp pointed knife and the portion 
which is lower than the elevation which the contour represents is 
removed. From these pieces the model can be built and the several 
parts glued together. 
Advantages of Orthographic Projection 
The chief advantages of this form of illustration may be summarized 
as follows: 
1. The amount and location of the injury throughout the planting 
are clearly shown. 
2. It permits the worker to use any degree of refinement and pre¬ 
cision in the illustration of his problem and the data may be taken with 
this point in view. 
3. When the data are plotted the chart assists in making proper 
deductions regarding the relationships between the distribution of the 
injury and the environmental factors. 
4. If desired, the worker can use the data for the more exact but 
more laborious methods of biometrical analysis. Correlation coeffi¬ 
cients and equations will give relationships in a quantitative form and 
can often be used supplementary to an orthographic chart. The point 
to be emphasized is that data taken accurately for this kind of chart 
can be used for more detailed analysis but the converse is not neces¬ 
sarily true. 
5. This form of chart can be made with comparatively little extra 
study of principles and methods. In this respect, it surpasses bio¬ 
metrical analysis for the average worker, since the latter type of 
analysis requires a rather extensive ground work in mathematics, in¬ 
cluding calculus, theory of probability and least squares, before one can 
use the methods with facility and precision. It is true that one must 
master the principles of topographic surveying but these are not very 
difficult. The acquiring of speed and accuracy in field work will, how¬ 
ever, require practice. To the worker who is not acquainted with the 
principles of topographic surveying, we would suggest that he first 
make a map of an area, in which the surface is sufficiently undulating 
to require considerable plotting to locate the contours. This prelim¬ 
inary exercise will assit him in plotting isofags where the positions of 
the contour points are not as obvious as in topographic surveying. 
6. The chart, if not too intricate, is readily comprehended. Even 
an intricate chart presents the conditions throughout every portion of 
