Monthly Microscopical 
journal, sept Law| Local Length of Microscopic Objectives. 149 
By Professor Zirkel, of Kiel. 
1. “ Microscopical Investigations of the Glassy and Half-Glassy 
Rocks.” —‘ Zeitschrift der deutschen geologischen Gesellschaft,’ 
1867, p. 737. 
2. “On the Microscopical Structure of Leucite and of Leucite- 
bearing Rocks.”—Jb., 1868, p. 97. 
3. “On the Microscopical Structure of Phonolite.” — ‘ Poggen- 
dorff’s Annalen,’ vol. cxxxi., p. 298. 
4. “On the Leucitic Rocks of the Erzgebirge.”—JD., vol. exxxvi., 
p. 044. 
5. “On the Microscopical Structure of the Lava of Nea Kammeni, 
near Santorin, erupted in 1866.”—‘ Neues Jahrbuch fiir Mine- 
ralogie, &.,’ 1866, p. 769. 
6. “ Investigations on the Microscopical Structure of Basaltic 
Rocks.” —A separate work. Bonn, 1870. 
By Professor Vogelsang, of Delft. 
1. “The Philosophy of Geology, and Studies on the Microscopical 
Structure of Rocks.” —A separate work. Bonn, 1867. 
2. “On the Nature of the Liquid enclosed in certain Minerals.”— 
‘ Poggendorff’s Annalen,’ vol. exxxvii., p. 56. 
By Professor Fischer, of Freiburg. 
1. “ A Chronological Survey of the gradual Introduction of the 
Microscope into the Study of Mineralogy, Petrography, and 
Paleontology.” —A separate pamphlet. Freiburg, 1868. 
2. “Critical Microscopical Mineralogical Studies.’—A_ separate 
pamphlet. Freiburg, 1869. 
By A. Kengott. 
“ Observations on Thin Sections of a Caucasian Obsidian.”—A 
separate pamphlet. St. Petersburgh, 1869. 
IX.—On the Focal Length of Microscopie Objectives. 
By Cuas. R. Cross. 
Tue investigation of which the present article is a summary, was 
undertaken in order to see if some reliable method of measuring 
the focal length of microscope objectives could not be found. The 
importance of such a method will be apparent to all who haye had 
occasion to make use of objectives by different makers. The focal 
length of lenses of the same denomination ig subject to so great a 
