用最明显的图形来描述这个过渡的特性就是：当在两个圆之间绘制一条直线，直线与每个圆只有一个交点时，那么这条线就是两个圆的切线。
The quality of this transition is best visualized by a straight line drawn between two circles: the line's only point of contact with each circle is tangential.
用轻细的6 H铅笔线绘制主要的中心线和视图大体的轮廓框。
Draw the main center lines and block in the general outline of the views with light, sharp 6h pencil lines.
想象一下你不用知道表面的颜色是什么，就可以用相同的画笔在白色的表面上绘制一条黑色的线并且在黑色的表面上绘制一条白色的线。
Imagine a way in which you could use the same pen to draw a black line on a white surface and a white line on a black surface without knowing what color the surface is.
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