http://en.wikisource.org/wiki/Popular_Science_Monthly/Volume_2/November_1872/Aims_and_Instruments_of_Scientific_Thought_I
http://en.wikisource.org/wiki/Popular_Science_Monthly/Volume_2/December_1872/Aims_and_Instruments_of_Scientific_Thought_II
http://www.uta.edu/philosophy/faculty/burgess-jackson/Clifford.pdf
http://www.archive.org/stream/lecturesessaysby02clifrich#page/n9/mode/2up
http://en.wikisource.org/wiki/Popular_Science_Monthly/Volume_2/December_1872/Aims_and_Instruments_of_Scientific_Thought_II
http://www.uta.edu/philosophy/faculty/burgess-jackson/Clifford.pdf
http://www.archive.org/stream/lecturesessaysby02clifrich#page/n9/mode/2up