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Afterglow 夕照，晚霞
Here's an experiment you can try: look at something bright relative to its surroundings, such as a lamp or a sunny window, for ten seconds without blinking. Then shut your eyes and cover them over with your hands.
你可以嘗試這樣的試驗：不眨眼盯著看相對周圍而言光亮的東西十秒鐘，比如，一盞燈或灑滿陽光的窗戶；接著閉上眼睛且雙手蒙住。
You will find that the lamp or window still seem to be hovering in front of you, quite visible at first and only gradually fading away. Yet your eyes are closed and your hands are blocking them completely — so where is the light coming from?
你會發現那盞燈或窗戶依舊浮現在眼前，最開始清晰得很，又慢慢消失。可是，你不是閉眼了嗎？況且雙手不是嚴實地蒙住了嗎？那么，光線從何而來？
The answer is that the afterglow light you see isn't really light at all. It's the result of residual activity of the nerve cells on the back of your eye, called the retina. In sensory psychology this effect is called a "positive afterimage".
答案是殘留的余光根本不是真正的光芒。其實這是視網膜（眼后的神經細胞）的痕跡作用。感官心理學將這種現象稱作“正后像”。
You can think of it as something like the vibration of a string after it has been struck. The player's finger is taken away, and yet the sound continues, because the energy she or he put into the string takes a little while to dissipate.
你可以將“正后像”視為撥動后琴弦的振動。彈琴者手指離開了琴，而琴音依舊。因為琴弦上的能量需要點時間消退。
It's a similar situation with your eyes. In order to respond usefully to such a relatively faint signal as light, the cells in the retina need to be very sensitive. Over-stimulate them and some will continue responding for a couple of seconds after the stimulus is removed.
視覺的情況也類似，視網膜細胞的感光十分敏感以便及時感知像光線般微弱的信號。受到過度刺激的視網膜細胞，部分在刺激后持續反應。
Of course, the only thing we ever "see", really, is the activation of our retinal cells — that's what our brains interpret as light from the outside world. So for as long as it takes for them to calm down, you will continue to see a faint glow of light — even though nothing is there.
當然，唯一我們“看到”的是視網膜細胞的激活也就是大腦解讀的：外界的光線。所以，只要刺激后需要時間平復，你就能持續看到微弱的燈光，雖然，實際上并不存在。
