The illusion picture, and right, Patrick finishing the trail (Picture: Patrick DeIvernois)

After completing the 2,200 mile Appalachian Trail, Patrick DeIvernois and three friends has a celebratory whiskey and snapped a picture.

At the time, they didn’t notice anything special about the photo – but it’s now gone viral because of an illusion.

There are four bottles – but on first glance, it looks like there are just three people enjoying them.

Can you spot the fourth arm?

Can you work out where the arm was? (Picture: Patrick DeIvernois)

With one of the friends wearing a camouflage jacket, his arm blends perfectly into the background and his glove blends into Patrick’s kilt.

It’s only when you take a second look that you can spot the fourth arm.

Some people said the fourth arm as ‘the person’ standing behind Patrick but again it’s your eyes playing tricks as that’s his backpack with a cover over it.

The picture was shared on Reddit and later Twitter and people were baffled.

Once they’d figured it out, some laughed that they’d never seen camo work before.

One said: ‘This is honestly the first time I’ve ever seen cameo print actually do its job.’