PHYSICS: a BARUS 
117 
female caste has been abolished and its reproductive function trans- 
ferred to gynaecoid workers, i.e., to forms differing from ordinary workers 
only in their ability to produce worker as well as male offspring. It is 
very probable that even this abolition of whole casts has been accom- 
plished by very slow and gradual processes and not by sudden varia- 
tions, or mutations. 
1 Wheeler, The Ants of the Baltic Amber, Kdnigsherg Schr. physik. Ges., 55, 1914, (1-142). 
^ZooL Jahrh. Ahth. SysL, Jena, 8, 1895, (774). 
^ Die Fossilen Insekten, Leipzig, 1908, (p. 1283). 
4 Log. ciL, (p. 1285). 
5 Bull. Geol. Soc. Amer., 20, 1910, (427-606), and Climates of Geologic Time, Carnegie Inst. 
Washington Pub. No. 192, (pp. 263-298). 
6 Dewitz, Zs. wiss. ZooL, Leipzig, 30, 1878, (78-105), PI. 5. 
7 British Ants, 1915, (p. 131 and 221), Fig. 50. 
8 Bull. Amer. Mus. Nat. Hist., New York, 21, 1905, (405-408), 1 pL; Ants, etc., 1910, (p. 99), 
Fig. 63. 
^Rev. Suisse ZooL, 10, 1902, (444). 
lom'i., 18, 1910, (27). 
^^Ark. ZooL, 9, 1915, (71). 
^^BioL Bull., 4, Woods Hole, Mass., 1903, (180). 
13 Acta Soc. EnL Bohemice, 5, 1908, (139-146), 4 figs. 
" British Ants 1915, (p. 220), Fig. 47. 
15/. Exper. ZooL, 8, 1910, (421), Fig. 7. 
Zs. wiss. ZooL, 114, 1915, contains a full bibliography of the author's papers on the 
Lomechusini. 
"Wheeler, Boston, Proc. Amer. Acad. Arts Sci., 51, 1915, (257). 
18 Arnold, Ann. S. Afric. Mus., 14, 1916 (195). 
REFRACTIVITY DETERMINED IRRESPECTIVE OF FORM, BY 
DISPLACEMENT INTERFEROMETRY 
By Carl Barus 
DEPARTMENT OF PHYSICS. BROWN UNIVERSITYi 
Communicated December 13, 1916 
1. Introductory. — Some time ago^ I made a number of experiments 
on the use of curilinear compensators in connection with the displace- 
ment interferometer. It is obvious that the curvature in such a case 
must be very small, so that single lenses for the purpose are not easily 
obtained. The use of a doublet of two lenses of the same glass but 
respectively convex and concave, meets the case fairly well, the neces- 
sary refracting power being obtained by spacing the doublet. Lenses 
of about one diopter each gave the best results, bringing out fringes 
of quasi-elliptic and hyperbolic symmetry in great variety. 
Later it appeared as if plates of different varieties of glass, as for 
instance crown and flint, if placed in the two interfering beams would 
