Many easy to use Kodaks up to 3.5 x 3.5 inch (9 x9 cm) size had fixed focus lenses. Cameras for larger pictures had to be focused with the help of a distance scale and a pointer. For the snapshooting photographist this was fine. He or she didn't want more.
The serious amateur however wanted to do better and focus his image on a ground glass. To focus the image he moved the lens forward or backward and watched the effect on the ground glass. In the video on the left you can see this. First the focus is on the pole close to the camera. Then the lens moves backward and the focus changes to the tree in the distance. After a few seconds the focus changes back to the pole in the foreground.
To make things more difficult: the image on the ground glass is always up side down. To give you a better idea about what is on the ground glass, I have rotated the video. This one you see at the right.
The video was filmed on the ground glass in a No. 4 Cartridge Kodak with plate back.