The photo finish is such an ingrained part of horse racing that it’s easy to forget that it didn’t exist for most of the sport’s history. The earliest mention of its use that I can find is an August 8, 1935 Brooklyn Daily Eagle article by W.C. Vreeland, a sportswriter and champion of the equine game, who urged for the installation of an “electronic eye” at every track. The story:

“Saratoga–From the time the racing season opened, in the middle of April, I had advocated a ‘camera eye’ to judge the finishes of all race courses in this State. After considering the advantages of such an arrangement which positively and accurately tells which horse pokes its nose in line with the winning post in front of his opponents, the members of the State Racing Commission have decided on an ‘electric eye,’ which will be adopted on Oct. 1.

The electric eye is a motion picture machine which makes a photo of horses in action, and at the same time makes a picture of a split-second hand moving across the face of a dial. The two pictures are made on the same film so that there can never be a question of what race is being filmed.

The horses appear along the top of the film, and the moving clock hand along the bottom. The actual picture of the race is never recorded in full, only the last furlong or sixteenth pole because the finishes only of races are matters of dispute. 

The camera must be placed high above the race course on which it is to be operated. It is installed far behind the judges’ stand so that the horses will appear all one size in the eye of the camera. The camera itself is equipped with a developing mechanism which is set in motion at the same instant that the camera starts recording.

The film is automatically developed, washed, dried and printed in less than five minutes’ time after the race. The electric eye will cost each racetrack about $300 a day. The commission recommends that each racetrack in the State adopt it as part of the ‘placing’ of the horses as they finish past the winning post.”