第二篇论文原文下载地址: https://www.researchgate.net/profile/Jon_Sporring/publication/8329656_Virtual_Trackballs_Revisited/links/09e4150aa525511b6b000000/Virtual-Trackballs-Revisited.pdf
在线论文地址：https://www.researchgate.net/publication/8329656_Virtual_Trackballs_Revisited
