50．During the 1940's electron microscopes routinely achieved resolutionbetter than that possible with a visible light microscope，while the performance of x-raymicroscopes resisted improvement．

(that指代前面的resolution，possible with a visible．1ight microscope作that的定语)

20世纪40年代，电子显微镜通常比可见光显微镜的分辨能力更强，而X光显微镜的表现却没有改进。

分句1：During the 1940…

分句2：that…

分句3：while..

1和2嵌套

1和3并列