[495] Nilson 1903.

[496] Darbishire 1897.

[497] Bitter 1901, p. 191.

[498] Krabbe 1891.

[499] Darbishire 1907.

[500] Tobler 1911², 11.

[501] Bitter 1901².

[502] Lindau 1895.

[503] Nilson 1903.

[504] Bitter 1904.