Now, with hindsight, one can see that  the important factors were A: most retinal problems specifically  affect  blue-yellow discriminations, and  few tests are designed to  attack these problems. B: the Farnsworth-Munsell is difficult to use and the errors in old people  ( who are the ones who mostly get ill) are so large that the test is reduced in value. C: Variants ( e.g. The Lanthony test ) which are easier to use and sometimes more sensitive, are not really quantitative. Colour contrast testing with a computer  avoids all these problems.