# What's the name and cause of this visual phenomenom? **URL:** \ **Category:** Factual Questions\ **Created:** [June 13, 2004, 3:10pm UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121 "2004-06-13T15:10:14Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Mr.Blue\_Sky](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@Mr.Blue\_Sky](https://boards.straightdope.com/u/Mr.Blue_Sky)\ **Post date:** [June 13, 2004, 3:10pm UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121/1 "2004-06-13T15:10:14Z")
When looking at certain stars, they seem to disappear. When you look just to either side, they seem to reappear. What is this called and what causes it? ---
**Author:** ![Finagle](https://avatars.discourse-cdn.com/v4/letter/f/b19c9b/32.png) [@Finagle](https://boards.straightdope.com/u/Finagle)\ **Post date:** [June 13, 2004, 3:14pm UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121/2 "2004-06-13T15:14:37Z")
It’s your blind spot ([http://littleshop.physics.colostate.edu/Blind.html](http://littleshop.physics.colostate.edu/Blind.html)). The place where the optic nerve connects to the retina. No visual sensors there. In general, it’s not noticeable because the eye jitters around all the time. ---
**Author:** ![Ale](https://avatars.discourse-cdn.com/v4/letter/a/f19dbf/32.png) [@Ale](https://boards.straightdope.com/u/Ale)\ **Post date:** [June 13, 2004, 3:17pm UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121/3 "2004-06-13T15:17:33Z")
Actually, the most probable explanation is that the bottom of your retina is not as sensitive to light as the area around it; the bottom is more densely populated with colour sensitive receptors, and the periphery with light only (“black & white”) receptors, which are more sensitive. So when a dim image is focused in the more sensitive receptors it appears brighter. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [June 13, 2004, 3:19pm UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121/4 "2004-06-13T15:19:40Z")
I’m not aware that it has a particular name, but the effect you’re seeing is probably caused by the arrangement of light-sensitive cells in the retina. There are two types, called rods and cones. Cones are sensitive to color, but need quite a bit of light to work. Rods are very sensitive to light, but impart no color information. Our eyes have a large concentration of cone cells in the central area of the retina, called the fovea. The fovea is relatively poor in rod cells. However, the periphery of the retina is jam-packed with them, and has a high sensitivity to even small amounts of light. It’s well-known in astronomical circles that if you wish to view a particularly dim star, you don’t look directly at it, you look at it with your peripheral vision. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [June 13, 2004, 3:20pm UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121/5 "2004-06-13T15:20:52Z")
> [@](#): > > The retina of the human eye contains light sensitive rods and colour-sensitive cones, and can be separated into two general parts. The center of the retina contains mostly cones, so is more colour-sensitive. This is the center of your vision, with which (for most of the time) you look at things. The outside of the retina contains mostly rods, so is more light-sensitive. This is your peripheral vision and, because it is more light-sensitive, is able to see fainter objects. Thus, the best way to observe faint objects in not to look directly at them. [Observing Faint Objects](http://www.st-andrews.ac.uk/~astrosoc/new_site/data/how2/faint.html) ---
**Author:** ![Mr.Blue\_Sky](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@Mr.Blue\_Sky](https://boards.straightdope.com/u/Mr.Blue_Sky)\ **Post date:** [June 13, 2004, 3:23pm UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121/6 "2004-06-13T15:23:51Z")
Thanks! ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [June 13, 2004, 3:26pm UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121/7 "2004-06-13T15:26:14Z")
“Averted vision” may be the term you are looking for. It’s not the name of the phenomenon, but it refers to the technique of looking slightly away from a faint object to see it better. (For reasons Ale and QED stated.) ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [June 13, 2004, 3:26pm UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121/8 "2004-06-13T15:26:24Z")
> [@Q.E.D.](#): > > I’m not aware that it has a particular name, but the effect you’re seeing is probably caused by the arrangement of light-sensitive cells in the retina. There are two types, called rods and cones. Cones are sensitive to color, but need quite a bit of light to work. Rods are very sensitive to light, but impart no color information. Our eyes have a large concentration of cone cells in the central area of the retina, called the fovea. The fovea is relatively poor in rod cells. However, the periphery of the retina is jam-packed with them, and has a high sensitivity to even small amounts of light. It’s well-known in astronomical circles that if you wish to view a particularly dim star, you don’t look directly at it, you look at it with your peripheral vision. This is the most likely answer to the OP question. For example, the Andromeda galaxy is reasonably prominent to the naked eye if you don’t look directly at it. As you get older and have a certain amount of macular degeneration. of the “good” kind, it gets more pronounced. My central vision in dim light isn’t really very good any more. I first noticed this one time at dusk when I was coming down the last fairway while playing golf. I saw my ball out of the corner of my eye so I looked at it. It disappeared. I looked away and there it was again. ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [June 13, 2004, 4:24pm UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121/9 "2004-06-13T16:24:17Z")
**Q.E.D.** has it right. **finagle** describes a real entity, but the blind spot deficit is far more subtle than the rod/cone color preception difference, and the blind spot rarely comes into play in the dark. QtM, MD ---
**Author:** ![Finagle](https://avatars.discourse-cdn.com/v4/letter/f/b19c9b/32.png) [@Finagle](https://boards.straightdope.com/u/Finagle)\ **Post date:** [June 14, 2004, 12:18am UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121/10 "2004-06-14T00:18:31Z")
> [@Qadgop the Mercotan](#): > > **Q.E.D.** has it right. > > **finagle** describes a real entity, but the blind spot deficit is far more subtle than the rod/cone color preception difference, and the blind spot rarely comes into play in the dark. > > QtM, MD I did some web searching and found a couple of anecdotes from astronomers who actually take advantage of their blind spot to block out bright objects in order to see dimmer adjacent objects. I guess this works best when viewing through an eyepiece. And here’s a third phenomenon – the [Troxler phenomenon](http://www.fortunecity.com/roswell/borley/49/visual.htm#Troxler). The retina can get fatigued looking at objects, causing the image to fade. You’ll only notice this if you’re consciously overriding the eye’s tendency to jitter slightly. ---
**Author:** ![umop\_ap\_sdn](https://avatars.discourse-cdn.com/v4/letter/u/dbc845/32.png) [@umop\_ap\_sdn](https://boards.straightdope.com/u/umop_ap_sdn)\ **Post date:** [June 14, 2004, 7:36am UTC](https://boards.straightdope.com/t/whats-the-name-and-cause-of-this-visual-phenomenom/250121/11 "2004-06-14T07:36:08Z")
ISTR that there are no rods whatsoever in the fovea, nor blue cones, so that the red and green cones (which are the smallest photoreceptors) are packed as tightly as possible.