OPTIC AXIAL ANGLE. 
'55 
The actual time consumed in this operation is not great, and the values 
obtained are approximately correct. The objection to its use lies chiefly 
in the subjective factor involved, namely, the skill required in drawing 
accurately the phenomena observed, and also in the nice adjustment of all 
parts of the instrument. 
The location of the optic axis A i is at all times more accurate and trust- 
worthy than that of H, owing to the indistinctness and width of the black 
axial bars near the margin of the field in consequence, partly, of the rota- 
tion of the plane of polarization of the obliquely incident light-waves. 
METHODS WITH THE DOUBLE-SCREW MICROMETER OCULAR; ALSO WITH THE 
COORDINATE MICROMETER OCULAR.* 
In seeking for more accurate and at the same time simpler methods than 
those of Professor Becke described above, the writer has used (in place of 
the usual single-screw micrometer ocular, with a movement in one direction 
only) a double-screw micrometer ocular with movements in two directions 
normal to each other. This was first constructed in the workshop of the 
Geophysical Laboratory (Plate 2, Fig. 2).f By its use it is possible to 
determine the position of any point in the interference figure accurately by 
means of two micrometer-screw readings, which correspond to rectilinear 
coordinates in the orthographic projection and vertical small circle coordi- 
nates in the stereographic projection. By means of the constant K of the 
microscope for each of these micrometer movements, V and 77, which must 
have been determined previously by means of known angle values (table 7, 
*Amer. Jour. Sci. (4) 24, 336. 1907; (4). 31, 97. 1911; Jour. Wash. Acad. Sci.. 1, 6o-j6i. 1911. 
tThe two movements of the double-screw micrometer ocular, // horizontal and I' vertical, are effected by 
fine micrometer screws reading accurately to 0.005 mm. The construction of this ocular is similar to that 
of the single-screw micrometer oculars, except that here two screws with corresponding movements are 
used in place of the single screw. O <-Ramsden ocular; S, small stop aperture to reduce errors of parallax. 
This ocular is now made by R. Fuess & Co., Stegli'z. Germany. 
