If the reading is still off the drum on the steep side with the +1.25 D. lens, a +2.25 D. lens can be held up in front of the instrument and the reading taken. Again it must be converted
There are devices made to hold the lenses in place instead of trying to hold them up by hand. These photos show small +1.25 D. and +2.25 D.lenses mounted in small rubber holders that fit snugly into the opening in the objective.
