What two ways did Mr. Jamison bring doubt to Mrs. Barrett's testimony?

Through his questioning of Mrs. Barnett, Mr. Jamison is able to reveal the lights didn't work, she wasn't wearing her glasses, the intruders were more than twenty feet away, she couldn't distinguish their features or discern their height. He also brought in the black stockings found in the trash outside the store, which Mrs. Barnett said she hadn't worn or thrown away, and cited the men who murdered her hisband may have been wearing them over their faces.