“ The trial judge charged the jury under the applicable Georgia comparative negligence statute [1] that if the plaintiff had been himself negligent, though less so than the defendant, his recovery 'would be reduced in proportion to the amount of default attributable to him.' The jury returned a verdict of $2,500 although there had been m dical expenses of $1,300, evidence of lost earnings of $12,000, and other damage alleged up to a total of $100,000. ”