Because of the large number of instruments offered for the training of the fusion faculty in cases of squint, it would seem to be of value to discuss the indications for the use of some of these instruments and the results to be obtained.
These instruments may be classified according to their construction into:
Instruments depending on the use of prisms to produce the superposition of images.
Instruments depending on the reflection by mirrors to produce the superposition of images.
Combinations of 1 and 2.
Instruments depending on the projection of light forms on a screen to produce the superposition of images.
The following list includes most of the instruments employed in the United States and England: