1. Footnotes have been renumbered and moved to the end of the paper. 2. Tables have been moved to avoid splitting paragraphs. In some cases the tables have been reconfigured or split in order to work better with e-readers. 3. Except as mentioned above every effort has been made to replicate this text as faithfully as possible, including non-standard punctuation, inconsistently hyphenated words, etc. 4. The original printed text is available from HathiTrust at: https://hdl.handle.net/2027/uc1.b3805680 (see pg 353). The outline “CLASSIFICATION OF NEBULAE” starting on page 3 lists N.G.C. 2117 as a type E7 nebula. The version of this paper printed in _Contributions from the Mount Wilson Observatory_ No. 324 lists N.G.C 3115. Otherwise the papers are essentially identical. Special notation for the plain text version: 1. Text enclosed by underscores “_” is italicised. 2. Text enclosed by macrons “¯” has a line draw over it. 3. Text enclosed in braces following an underscore “_{ }” is subscript. 4. Text enclosed in braces following a circumflex accent “^{ }”is superscript. If only a single character, the braces may be omitted. 5. Text enclosed in heavy angle brackets “❰ ❱” is bold. 6. Text enclosed in dollar signs "$" is in TeX notation.