# The Math Behind the Classic Eye Chart Is Surprisingly Complex

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Next time you're forced to take a vision exam at the DMV, take a moment to appreciate the complex math that went into the eye chart. What seems like a fairly straightforward way to assess eyesight is actually the result of specific calculations that can tell you a lot about how the human eye works.

As The Verge explains in the video below, eye charts measure one aspect of healthy vision: visual acuity. This is our ability to make out fine details in our surroundings—kind of like the resolution of a computer screen, but instead of pixels, it's measured in degrees. It's easy for our eyes to tell the difference between two points of light coming from different directions, but if those points start to move closer together they will eventually blur into one. The angle created just as two lines of light become too close for our eyes to distinguish them is called the resolution limit. In healthy adults, it measures one-sixtieth of a degree, or one arcminute wide.

When a doctor asks you to read an eye chart, the resolution limit is what they're looking for. The letters in the middle of an eye chart are all designed to be exactly one arcminute thick. If your vision is sharp, you should be able tell the difference between the white spaces and the black lines of the text from 20 feet away.

A perfect 20/20 score on an eye chart test doesn't mean you have perfect vision: Visual acuity, along with color, contrast, and depth perception, are all important parts of healthy eyesight. And a higher resolution limit isn't always a sign of a permanent problem: For people who spend their days staring at a screen, it may be caused by the eye fatigue brought on by Computer Vision Syndrome. If this is a problem for you, here are some ways to tweak your behavior.

[h/t The Verge]

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High levels of the stress hormone cortisol can wreak havoc on your body. (According to a recent study on middle-aged adults, stress not only impairs memory but may even cause the brain to shrink!) Thankfully, some commonly dreaded activities can help reduce your frazzled state.

## 1. Washing the Dishes

According to a 2014 study published in the journal Mindfulness, a “mindful” approach to dishwashing could reduce stress. “A sample of 51 college students engaged in either a mindful or control dishwashing practice before completing measures of mindfulness, affect, and experience recall,” the study states. “Mindful dishwashers evidenced … increases in elements of positive affect (i.e., inspiration) [and] decreases in elements of negative affect (i.e., nervousness)." In other words, with the right mindset, zoning out in front of a sudsy sink is basically Nirvana.

Research suggests that clutter is more likely to stress out women. In 2010, a study in the Journal of Personality and Social Psychology looked to see how married couples dealt with (and felt about) messy homes [PDF]. “The wives in the study who perceived themselves as having a cluttered home or a home that needed work tended to have increased levels of cortisol throughout the day,” Emilie Le Beau Lucchesi wrote in The New York Times. "Those who weren’t feeling cluttered, which included most of the men in the study, had cortisol levels that tended to drop during the days.” So tidy up!

## 3. Exercising In A Group

Working out can feel like a chore, and exercising with a group can be a tad embarrassing—especially if you’re not on the same fitness level as everybody else. But according to research in the Journal of the American Osteopathic Association, exercising with a group is more beneficial at reducing stress than working out alone. “Researchers found that working out in a group lowers stress by 26 percent,” according to the press release. Go ahead and book that spin class!

## 4. Sniffing Your Partner’s Laundry

No sane person puts “sniff your significant other's dirty socks” on their to-do list, but perhaps they should. A 2018 study published in the Journal of Personality and Social Psychology shows that sniffing a loved one’s clothes can reduce stress. In the study, 96 women sniffed one of three scents—a neutral smell, their romantic partner’s scent, or the scent of a stranger. The stranger’s smell caused cortisol to spike. But their partner’s smell? It reduced stress.

## 5. Dwelling On Your Failures

The title of this study, which appeared in the journal Frontiers in 2018, says it all: “Writing About Past Failures Attenuates Cortisol Responses and Sustained Attention Deficits Following Psychosocial Stress.” According to the study, “[W]riting about a previous failure may allow an individual to experience a new stressor as less stressful, reducing its physiological and behavioral effects.” It sounds paradoxical, but the next time you're facing a crazy situation, just reflect on a time when it all went wrong—and things might not feel so bad.

## 6. Singing For All to Hear

For the shy and tone-deaf, singing in a group might be a anxiety-fueled nightmare—but they should try it anyway. A pilot study presented at the Society for Neuroscience conference last year showed that, in people with Parkinson's disease, singing in a group can reduce heart rate, blood pressure, and cortisol levels. (Researchers cautioned that this is preliminary data.) The findings jibe with a 2016 study from Drexel University that found, no matter your skill level, making art usually reduces cortisol levels [PDF].

# Nearly Half of American Adults Don't Know Their Own Blood Type

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If you know your blood type, you’re better off than nearly half of Americans. Of the 1004 adults in the U.S. who answered a recent Quest Diagnostics survey, only 57 percent said they knew whether they have type A, B, AB, or O blood.

This is roughly the same number of people who could recall their childhood phone number (55 percent), the survey revealed. By comparison, 74 percent of respondents remembered their lengthy Wi-Fi password, and 75 percent knew how much money was in their bank account.

For many, other personal health information was even murkier. Fewer than two in five people knew their cholesterol or blood sugar levels. Considering that these details provide important insights into one’s risk for certain diseases, survey administrators said this is a cause for concern.

“With consumers increasingly engaged in their own and their loved ones’ health care, it’s critical that they ‘know their numbers’—and have those numbers readily accessible—to ensure productive communication with their healthcare provider for both routine and critical care,” Cathy Doherty of Quest Diagnostics said in a press release.

With the exception of emergencies, at which time you may receive a universal donor's O-negative blood, doctors will almost always conduct blood typing and cross-matching tests to determine your blood type and identify minor antigens in your blood before conducting a transfusion or surgery.

It’s still important to know your blood type for other reasons. Newborn babies, for example, can develop hemolytic disease if their Rh blood type (meaning whether it's positive or negative) doesn’t match their mother's. And depending on your blood type, you may also have an increased risk for blood clots, heart disease, certain types of cancer, and even severe diarrhea and mosquito bites.

If you know your blood type, you’re also in a better position to donate blood and help people in need if there’s a natural disease or emergency, or if blood banks simply have a low supply. O blood tends to be the highest in demand (and O-positive is the most common blood type), but blood banks may issue public notices from time to time if they need a particular type.

If you’re unsure of your blood type, clinical labs like Quest Diagnostics offer blood type tests. You can also order test kits online from Amazon.

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