by now与so far的区别

so far
是表示until this time or to this place到现在为止；到这里为止，后面的句子一般用完成时，而且so far后面一般接比较贬义或者消极的信息，比如说
Fusion has not, so far, been tamed.
到目前为止，还不能控制聚变。
So far no further news had been obtained．至今没有得到进一步的消息
by now
也是表示到目前为止，但是不像so far后面没有要加消极信息的那一层意思
By now he has dropped the complacent notion.
现在，他抛弃了一个得意的想法
By now he had stirred at best only a partial sympathy.
他这时候的心情，也至多只能说是略有几分同情而已。