434 
PHYSICS: C. BARUS 
If the white slit images in the telescope pass through each other (in 
consequence of the rotation of M' about a vertical axis, as specified) 
the direction of fringes twice changes sign in rapid succession and this 
probably occurs when the white slit unages are coincident. Barring 
this inversion, the march is regular and proportional to the rotation. 
With the displacement (AiV) if the mirror M' on the micrometer 
screw normal to its face, the fringes pass through a continuous succes- 
sion of color schemes, but soon vanish; for they coincide in adjustment 
with the centered spectrum fringes. Similarly if a pair of mirrors {MM' 
or NN' (fig. 1) ) rotates about a vertical axis as a rigid system, the same 
continuous change of color scheme and evanescence is apparent. 
These interferences differ, naturally, from the spectrum interferences; 
they also differ from the achromatic interferences, which are much finer 
fringes, partaking of the regular fringe pattern seen with biprisms. 
.They are a separate phenomenon, quite sharp and definite, occurring 
under like conditions of adjustment, but under different conditions of 
observation (ocular out of focus and fine slit). In the principal focus 
the two sharp extremely bright slit images are alone present. They are 
absolutely identical in structure, however, and their spectra when super- 
posed would interfere symmetrically throughout their extent. Under 
these circumstances the rays intersecting in the white slit images also 
interfere before and behind the principal focal plane of the telescopic 
images specified, and this interference is not destroyed when the slit 
images are separated (rotation of opaque mirror M' about vertical axis) , 
or when the slit images are passed through each other. What is not at 
once seen however is the reason of the occurrence of large sharp definite 
hyperboHc forms instead of the usual Young or Fresnel fringes of two 
slits or slit images. 
On the Michelson interferometer these fringes (like the achromatic 
fringes) are extremely faint and can hardly be detected except by put- 
ting them in slow motion. The spectrum fringes are equally strong in 
all cases. It appears therefore that the two half silvers are favorable to 
evolving both the hyperblic and the achromatic set of fringes. The 
Michelson design is thus not useful here, nor for the measurement of 
small angles of rotation by the methods described, as the mirrors would 
have to be rotated in opposite directions. 
Further work with the complementary fringes on different interfer- 
ometers of the Jamin type showed that to produce the hyperbolics, the 
fine sht images must coincide horizontally and vertically. They do not 
in this case probably coincide in the fore and aft direction; for the plates, 
etc., were not optically flat. When the slit images are separated at the 
