某口袋中装有红色、黄色、蓝色三种颜色的小球（小球出颜色外完全相同）共60个．通过多次摸球实验后，发现摸到红球、黄球的频率分别是30%和45%，由此估计口袋中蓝球的数目约为____ 个．

    Before discussing different kinds of emotions,let us briefly talk about how researchers measure bodily processes and action or behavior,and how this relates to what we do in our daily lives when we observe emotions in others.

    Bodily processes can be directly measured by means of a polygraph.When a polygraph is skillfully used to compare how we react bodily with what we are saying,it is called a “lie detector”.Bodily processes can also be measured indirectly.This is what we do when we observe someone blushing (脸红).However,we are not always aware of what bodily processes respond to.

    Measuring action or behavior is the other way researchers assess the emotions.For example,one measure of fear of snakes is how close a person will go to the snake.Another procedure is to have a person tell how afraid he is,or how he feels.In this way,researchers have developed the so－called “fear thermometer” to assess a person's fear.In our everyday living,we do very much the same thing.Only not too systematically,we react to what a person does, what he says,how he says it,and how he looks.Is he smiling? Is his voice trembling? We put all this observations together to infer what a person is feeling.

    However, we do not always act as we feel.Sometimes we do things that we don't feel like doing.Sometimes we say we feel one way and then we act another.Actors,for example,successfully learn to “make believe” emotions,or learn to hide them.Thus we cannot always tell what a person is feeling by what he says or by what he does.

阅读下列材料，结合所学知识回答。
对历史人物的评价摆脱不了政治环境的影响。综观国共关系史，随着两党关系的演变，共产党对蒋介石的评价也有不同。自西安事变以后，共产党对蒋介石的态度逐渐发生变化，特别是1937年到1945年期间。如：1938年在延安六届六中全会上，***就说过“国民党历史上有两个伟大的领袖，第一个是孙中山，第二个是蒋介石”。1945年。共产党也公开表示，“承认蒋先生的领导，承认蒋先生在全国的地位。”但是，在1946年到1949年期间，共产党对蒋介石的态度又发生变化，斥之为“人民公敌”、独夫民贼”。还有大众流传的“蒋该死”、“蒋匪帮”之类，就是那个时期出现的。
问题：
（1）分别结合1937年到1945年以及1946年到1949年的主要史实，简要分析共产党对蒋介石评价高低的变化及原因。
（2）你认为评价历史人物的基本原则是什么？