258 SUMMARY OF CURRENT RESEARCHES RELATING TO 



about one three-millionth of an inch, and as, of course, one five-millionth 

 would make a difference of one-fiftieth of an inch in the focal length of 

 lenses of certain curvature, this error, though mechanically small, is 

 optically considerable. At the same time, the author is doubtful whether 

 a spherometer of greater accuracy could be constructed, and even in that 

 event he thinks the personal and temperature errors would probably 

 nullify the advantage. 



F.R.M.S.— Visibility of Minute Flagella. 



English Mechanic, lxxx. (1905) p. 527 (4 figs.). 



Zeiss, C. — Stereoscopy: Pulfrich Stereo-Comparators. 



[A catalogue by tbe Jena firm of this valuable instrument, which 

 is especially applicable to the purposes of Stellar Astronomy, 

 Metronomy, Observations of Sun and Moon, Meteorology, 

 Geology, Topography, Photogrammetry, etc.] 



Jena, 1903, 16 pp. 



The following reprints of pamphlets by C. Pulfrich bearing on the 

 Stereo-Comparator have been also published by Julius Springer, Berlin ; 

 they are extracts from the " Zeitschrift fur Instrumentenkunde " : — 



1. TJeber einige stereoskopische Versuche. 



August 1901, pp. 221-4 (1 fig.). 



2. TJeber eine Prufungstafel fur stereoskopiscb.es Sehen. 



September 1901, pp. 249-60 (1 fig. and 1 pi.). 



3. TJeber neuere Anwendungen der Stereoskopie und iiber einen hierfiir 



bestimmten Stereo-Zomparator. 



Marcli, 1902, pp. 65-81 ; May, 1902, pp. 133-41 ; 

 June 1902, pp. 178-92 (15 figs.). 



4. Neue stereoskopische Methoden und Apparate fur die Zwecke der 



Astronomie, Topographie und Metronomie. Part I. 



[This is practicallv a collection of all the previous articles.] 

 J. Springer (Berlin, 1903) 69 pp., 27 figs. 



5. TJeber einen Versuch zur praktischen Erprobung der Stereo-Photogram- 



metrie fur die Zwecke der Topographie. 



November 1903, pp. 317-34 (2 figs.). 



6. TJeber die Anwerdung des Stereo-Komparators fur die Zwecke der 



topographischen Punkbestimmung. February 1904, 4 pp. 



Other reprints, also obtainable through O. Zeiss, on Stereoscopy, are : — 



1. C. Pc lfrich— TJeber die bis jetzt mit dem Stereo-Komparator auf 



astronomischen Gebiete erhaltenen Versuchsergebnisse. 



Reprinted from V. J. S. der Adron. Gesdl. Jahr. 37, 

 9 pp., with a stereogram of the moon. 



2. Von Hvbl, A. F. — Die Stereophotogrammetrie. 



Reprinted from Mitt, des K. u. K. 

 Militdrgeogr. Inst., Band xxii., 16 pp. 



3. „ „ Die Stereophotogrammetrische Terrainaufnahme. 



Op. cit., Band xxiii , 30 pp.. 6 figs., and a 

 stereogram of a mountain landscape. 



