Reprinted from Chapter 8, Bits, Boots and Bridles by Joseph C. O'Brien in Care and Training of the Trotter and pacer by James C. Harrison, published by The United States Trotting Association, 1968
Let's say you have a horse that is turning his head to the left. You put the Murphy blind on the right eye. Then, when the horse turns his head to the left, the Murphy blind, if adjusted properly, obstructs the forward vision of the right eye. It is natural for a horse to want to see what is in front of him and consequently the Murphy blind serves as an incentive for him to turn his head back toward the center and hold it straight.
The murphy blind is a valuable aid helping a horse to carry his head straight. The blind, left, (the Murphy blind is on the right eye) must be critically adjusted so that the horse is forced to hold his head absolutely straight out in front of him in order to have completely unrestricted forward vision, which, of course, all horses desire. Illustrated is a horse that is inclined to turn his head to the left and it will be readily observed that if this horse turns his head to the left he will lose forward vision out of the right eye. The proper side adjustment is shown at the right in a drawing that also illustrates a Murphy blind with a "window". The "window" provides backwards vision for a horse that requires both a Murphy blind and an open bridle.
In putting a Murphy blind on a horse, do not put it on so that the eye is completely blinded because that would be defeating the purpose. If the eye were completely covered and the horse couldn't see straight ahead out of that eye no matter which way he turned his head, he would still turn his head to the left as he had been doing.
The Murphy blind should be adjusted so that it extends out in front of the horse's eye to a point where the horse must keep his head straight in order to have an unobstructed forward view out of that eye. As long as a horse keeps his head straight he has complete forward vision out of both eyes. It does not take a horse long to learn this and most of them will soon be going straight.
A Murphy blind is easier to put on and adjust with an open bridle, but can be used with any kind of bridle. Always use a nose strap to keep the sides of the bridle from sagging or pulling back when you pull on the lines. Without the nose strap, the sides of the bridle will come back when you pull on the bit and pull the Murphy blind back against the horse's eye.
I always use wire to fasten the upper and lower corners of the front of the blind to the front part of the cheek. Then I can set the blind at whatever distance I want it from the front of the eye and be sure it will stay there. If you use a shoe lace or something that is not stiff to fasten it to the cheek, the blind will flop in and out and won't stay at the angle at which you have adjusted it.
Once in a while you will run across a horse that requires an open bridle and a Murphy blind as well. Shadow Wave was such a horse. He needed the blind because he had a habit of turning his head but he also needed the open bridle because he would have a tendency to pace the front end too much if he couldn't see the horses behind him.
So in order to put a Murphy blind on him and still have what amounted to an open bridle, I cut a little window in the Murphy blind (see fig), right behind the eye, so that he could see behind him out of that eye. It worked very well and I recommend it for horses of this type.