While holding Alt create another elliptical selection so that you would subtract the original selection to leave you with the thickness of the outer button's light as shown in the illustration below.
在创建另一个 椭圆选区 时按住 Alt 键，这样就能用这个圆减去原来那个外圆，于此你就得到下面展示的示意图那样的按钮灯外环厚度。
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Observations by Hubble’s Advanced Camera for Surveys spotted 29 dwarf elliptical galaxies in the Perseus Cluster located 250 million light-years away and one of the closest galaxy clusters to Earth.