NavList:

I had the same problem with my  vernier sextant of unknown vintage.

I cleaned the magnifier and the diffuser with eyeglass cleaning fluid. Then I discovered by accident that it was a focussing problem. When I gently pulled the magnifier away from the arc the vernier became very clear. After several careful pulls the magnifier is now in the correct position.

To read the vernier hold the sextant so the arc  is perpendicular to the light source and tilt it so that the light glances across the graduations. The  light should pass through the diffuser (although I understand that the diffuser is missing from your sextant).

Do not give up. A few weeks ago when I decided to use my sextant seriously I despaired. The sun was extremely faint. It appeared when I cleaned the mirrors and the shades. Then, as described above, I could not read the vernier. Now I am enjoying taking noon sights, ex-meridians, equal altitudes and double chronometers. Occasionally I even  work out a boring intercept (-;

It is satisfying to prove that GPS and Google earth are reasonably - to within 0.5'  !!!!!!!!!!!!!!