PHYSICS: C. BARUS 
435 
same horizontal level into two fine parallel lines, the complementary 
fringes in fact become Fresnellian fringes, finer as the sHt images are 
more separated and as the ocular is more rearward or forward. This is 
precisely what should occur. We may conclude therefore that the com- 
plementary fringes are Fesnellian interferences of two slit images and 
that the central hyperbolic forms are due to outstanding front and rear 
positions of the two slit images, which seem to coincide in the field of 
the telescope. Differentiated from these, the achromatic fringes are 
referable to the colors of thin plates. I have in fact since succeeded in 
obtaining the complementary fringes in the shape of broad straight gor- 
geously colored vertical bars, without suggestion of hyperbolic contour. 
An attempt was made to get quantitative estimates of the passage of 
fringes, on rotating the paired mirrors NN' over an angle a. To control 
the small angles the device figure 4 was improvised and did good service. 
Here e is the tangent screw of a 6" divided circle. It is surrounded by 
a snug anulus of cork and holds the brass ring /, on whose surface a 
coarse screw thread has been cut. Near this and with its axis in parallel 
is a J inch screw a and socket (not shown), controlled by the disc h. A 
strong linen thread c is looped once around / in the grooves of the screw 
and once or twice around the grooves of a, the string being normal to 
the cylinders and kept taut by two small weights, g, about a half ounce 
and hy about an ounce. The head h may be turned either way and the 
angle read off in minutes on the head of the tangent screw e. 
The theoretical value apart from glass paths and other corrections 
should be, per fringe vanishing 
2R^a = X 
where is the radius of rotation corresponding to the angle Aa and X 
the mean wave length of light. In the given adjustment 2R = 10 cm. 
was the normal distance apart of the two interfering beams. Hence 
60 X 1Q~^ 
10 
= 6X 10"^ radiaas or 1.2", 
