56 
PHYSICS: C. BARUS 
zontal. The shift of achromatic fringes due to micrometer displacement may 
therefore be at once expressed in terms of the spectrum fringes rigorously in a 
given wave length. 
A very interesting transformation of the design, figure 2, will be noticed, 
if by rotating any of the mirrors, iV, w, figure 1 for instance, the two white 
slit images seen in the telescope, are passed horizontally through each other. 
During this motion the originally vertical, nearly linear achromatic fringe, 
passes through the form of the area between the hyperbolae a and ^, figure 2 ; 
next through the area between b and c (coincidence of slit images) ; then into 
the area between c and d\ finally again into a vertical hair line, always re* 
taining its individuality among the surrounding colored fringes of similar 
shapes. The whole is particularly vivid if the fringes are observed with 
the ocular drawn well forward, quite out of focus. The same may be done 
by adding a diopter spectacle lens (convex or concave), to the objective. The 
bearing of this will be seen presently. 
3. Curved compensators. — If the rays 85, 21 in figure 1 are brought to coin- 
cidence, it is obvious that a lens, either convex or concave, may be inserted 
between the mirrors m and m' and normal to the rays, without destroying 
the interferences, though they must be greatly modified in form. If the 
lens is symmetrically inserted, the two broad slit images will be equally wide, 
so that coincidence is perfect. The fringes so obtained are usually large, 
briUiantly colored circles. In case of imperfect plate they become oval and 
coarse. The large central disc is achromatic. To center the fringes the 
mirror m' may be rotated on a vertical and horizontal axis, until the symmet- 
rical circular figure is obtained. Here again the individuality and even the 
approximate position of the achromatic fringe is retained on passing the broad 
slit images through each other; but the sequences of types of fringe is peculiar. 
As the slit images separate toward the right or the left, because of the corre- 
sponding rotation of m on a vertical axis, the originally colorless disc of the 
circular fringes not only moves to the right or left, but at the same time 
becomes very vividly colored. The coarse fringes now show considerable 
resemblance to the coronas of cloudy condensation, in which there is also a 
colored disc. When the slit images have been markedly separated, the disc 
vanishes and thinner lines appear, at first as complete circles surrounding the 
fading disc, but rapidly losing curvature to become vertical. Throughout 
the whole transformation there has been a grouping of symmetrically con- 
centric colored circles, on both sides of the achromatic circle. To state this 
differently: each originally linear fringe, in turn, on expanding (slit images 
approaching coincidence), contracts vertically and broadens horizontally 
into a disc, which retains the color of the fringe out of which it originates. 
The same result may be obtained by moving the lens inserted between m and 
m' into both rays, fore and aft (direction 8, 5). Similarly the corresponding 
sequence between horizontal fringes appears, on moving the lens up and down. 
If the nij m', mirrors are moved bodily fore and aft, however, the circular 
