This measurement can also be made with a goniometer eyepiece, in which a row of parallel double-marks are used instead of the cross threads.
He described the reflecting goniometer in 1809 and the camera lucida in 1812, provided microscopists with the "Wollaston doublet," and applied concavo-convex lenses to the purposes of the oculist.
The fact they blinded the goniometer was significant, as they reduced bias.
Another negative aspect is that when the testers took the measurement they gave the goniometer to the model to record their readings.
All subjects used the same goniometer for all three measurements.