I found a high resolution scan of this image with descriptive text below the image, published in the 1936 National Geographic, on the Geographicus website. This scan does not suffer from vertical distortions. The shown curvature of the horizon is smoother than on the image above. Although they don't give specifications of the camera lens used, they state that this image shows the actual curvature of the earth. The horizontal line already appears in the National Geographic image and was placed there to better show the curvature.
There are no atmospheric distortions, as The significance of the photograph is that both the camera and the target (the top of the "dust sphere") are in the stratosphere, that is, the line of sight is wholly through the stratosphere. Ninety-six per cent of the earth's atmosphere was below the camera when this picture was made at 72,395 feet. With only four per cent of earth's atmosphere above this height, and that essentially free from dust, there is little to scatter sunlight; consequently the upper sky is very dark.
Because of the filter that was placed in front of the camera lens and the particular type of infra-red-sensitive film used, no visible light acted on the film, and the photograph resulted wholly from light that the eye could not see.
The image above suffers from vertical stripes caused by the scanner, that introduces slightly vertical distortions. This stripes are not present in the original image from National Geographics. The horizontal line below the horizon appears in all copies of this image I have found in the internet. If compressed horizontally, the line in the image above shows a very slight concave curve of anout the width of the line. So the curvature of the horizon would be slightly more pronounced in the original image.
|Image URL|| https://3dnews.ru/|
|Website||3DNews Daily Digital Digest|
|Article||A frozen moment: historical photographs taken for the first time|
|Authors||Sergey and Marina Bondarenko|
|Translation to en|| https://translate.google.com/|
|Note|| This is the biggest and best image version I could find with https://tineye.com/|
|Other Versions|| https://tineye.com/|
|Upload||Dienstag, 22. Dezember 2020; wabis|
|Dimension||1600 x 944 Pixel|
3DNews Daily Digital Digest ( 3dnews.ru ) is the first independent Russian online publication dedicated to digital technologies. Founded in 1997. The goal of the project is the timely publication of the most objective information about everything that happens in the IT market, as well as help users of digital devices in choosing, purchasing and the most effective use of equipment and software.
Source (Translation google): https://translate.google.com/
This event took place in 1935, and the National Geographic Society was again one of the main sponsors. With the Explorer-2 balloon, which climbed to a record height of 22,066 meters for the time, two balloonists, Albert W. Stevens and Orvil A. Anderson, took a picture showing the bend our beautiful planet.
Explorer II was a manned U.S. high-altitude balloon that was launched on November 11, 1935, and reached a record altitude of 22,066 m (72,395 ft). Launched at 8:00 am from the Stratobowl in South Dakota, the helium balloon carried a two-man crew consisting of U. S. Army Air Corps Captains Albert W. Stevens and Orvil A. Anderson inside a sealed, spherical cabin. The crew landed safely near White Lake, South Dakota, at 4:13 pm and both were acclaimed as national heroes. Scientific instruments carried on the gondola returned useful information about the stratosphere. The mission was funded by the membership of the National Geographic Society.