.//*[@id='rg_s']/div[1]/a/@href

http://www.google.com/imgres?imgurl=http://images.enstarz.com/data/images/full/15421/emma-watson.jpg&imgrefurl=http://www.styliwallpapers.com/celebrities/emma-watson/&h=2332&w=3500&tbnid=DPhW7CJ1erAD0M:&zoom=1&docid=22MKylYlja8LIM&ei=9oOUVbzdHsShgwTXqYOYBw&tbm=isch&ved=0CBsQMygAMAA

.//div[@class='rg_di rg_el ivg-i'][1]/a[@class='rg_l']/@href

>response.xpath(".//*[@id='rg_s']/div[1]/a/@href")
# returned: []
>response.xpath(".//div[@class='rg_di rg_el ivg-i'][1]/a[@class='rg_l']/@href")
# returned: []

>response.xpath(".//title/text()").extract()
# returns: [u'emma watson - Google Search']