The goal in operation for concomitant squint or in orthoptic training should be to establish balance of the extra-ocular muscles so that when the visual axes are fixed at infinity there is no excessive pull on any muscle or muscles in order to maintain fusion.
To date there have been only a limited number of instruments worthy of consideration which are sufficiently accurate for making such measurements of the different angles of reflection and deviation as will furnish basic facts on which laws can be established. The Stevens tropometer is the only instrument which has been used to determine the relative strengths of the individual rectus muscles. In this instrument a tangent scale was used, which could be set at variable distances from the eye to be measured. This necessarily gave faulty readings by reason of the varying distances of the scale from the eye.
It is believed
Park GE. THE PRECISION ANGLOMETER. Arch Ophthalmol. 1936;15(4):703–709. doi:10.1001/archopht.1936.00840160127010