COLOR SCHEME. 23 



A second principle follows from the requirement of economy in printing: The 

 color scheme should be devised to accomplish the required distinctions with the 

 least practicable number of impressions or printings. 



As the lithographic press prints but one color at a time, it might seem that 

 there must be as many printings as colors; but printing one color over another 

 has the effect of combining them as if they were mixed and therefore a small number 

 of printings may yield many distinctions. In colored illustrations three colors — 

 red, yellow, and blue or green — usually suffice, reinforced sometimes by black. 



On assuming that we proceed with the three primaries, it is evident that by 

 overprinting we may obtain the three secondaries and also one combination of all 

 three primaries in the full strength of their several hues. This combination wiU 

 be almost black. These seven effects are strong unmodified hues. To produce 

 tints the lithographer substitutes fine lines or dots in place of flat colors. Many 

 of the patterns employed are almost invisible, but they permit the white of the 

 paper to shine through and lighten the hue. Several distinct tints of any one 

 primary hue may be produced at one printing, and their various combinations con- 

 stitute a wide range of secondaries and tertiaries. Only an experienced lithographer 

 can know in advance what the effects of the combinations will be and how distinct 

 they will be. They fall into groups and of each group some are too closely similar 

 to one another for practical distinction and others are undesirable because they are 

 distasteful. But a large number of good distinctions is possible with three print- 

 ings, the number is very greatly increased by a fourth printing, and only very 

 numerous and very refined differences require many printings. The geologic colors 

 on the map of North America are produced by 12 printings, which yield 42 distinct 

 effects. Two more printings are required for the black and blue of the base map. 



Conspicuous patterns are more or less used in geologic maps and greatly increase 

 the range of recognizable distinctions. They are, however, a concession to tech- 

 nical requirement and offend good taste unless they are very skillfully designed. 



The third requirement, that of good taste, is commonly little considered, but 

 if it is satisfied the prime requirement of proper distinctness will also have been 

 met; for the essential of good taste is adaptation to purpose. In general it may 

 be said that pleasing maps result from an association of light tints, particularly 

 light tints of tertiary hues in large areas, with appropriate contrasts of bright 

 primary or secondary hues which may be proportionately more brilliant as the 

 areas covered by them are smaller. 



USAGES. 



If entire freedom of choice and arrangement in devising a color scheme is 

 allowed, the principles stated above may easily be applied in most cases to produce 

 a thoroughly satisfactory map ; but usage has established certain associations of 

 colors with definite meanings, and it is misleading to contravene those associations. 

 Whether the difficulty thus occasioned in the mind of a reader is such as to over- 

 balance the advantages that may be gained by using colors arbitrarily instead of 

 according to custom is determinable only with reference to each particular case and 

 is then often a matter of personal judgment. 



