PHYSICS: C. BARUS 
125 
The active slit in this experiment is the image within the cylinder, of the 
slit of the collimator and the former is sufficiently fine to show the Fraunhofer 
lines, even when the latter is a milhmeter broad, so that there is no deficiency 
of light. 
But in relation to the detection of the interferences, the two reversed spectra, 
strongly divergent in their homogeneous rays, introduce certain grave diffi- 
culties. For it will appear that the spectrum issuing at d\ figure 1, passes 
over the distance b further than the spectrum issuing at d, before they reach 
the telescope together. The result is that the apices of two spectra lie in 
different focal planes, unless the telescope T is very remote. This makes the 
adjustment difficult. 
To obviate this annoyance a symmetrical adjustment, with an additional 
mirror at d, figure 1, corresponding symmetrically to TV and a symmetrically 
placed cylinder G, is here preferable. In such a case the spectra lie in the same 
focal plane, and since they have undergone 2 and 3 reflections, respectively, 
before reaching T, the interferences of non-reversed spectra are obtained 
without much difficulty. In my experiments, owing to the irregularity of the 
glass cylinder used, the fringes were correspondingly irregular; but otherwise 
clear and strong, as a wide slit is admissible. 
The case of two internal reflections is complicated by the occurrence of 
multiple images from N, figure 3, even when one side is half silvered. This is 
particularly the case when the cylinder G contains water, as in my first exper- 
iments; for the glancing angle at b is then but 25°. There is an advantage, 
however, inasmuch as N may be placed at a correspondingly .large distance 
from G. In spite of the duplicated images, the fringes were found more easily 
and were less irregular than anticipated. They are liable to be reproduced 
usually in a different size and orientation in each of the images. They will be 
found in the colored edge and even in the white glare (caustic) which emanates 
from the cyUndrical surfaces. They could be made quite large, clear and 
strong, moreover, although the cylinder used was (as above) an ordinary 
glass shade. As in case of the triangular interferometer, the fringes rotate 
when TV is displaced parallel to itself on the micrometer screw. To control 
their size TV is to be rotated on a horizontal axis. 
^ Advance Report from the Carnegie Publications of Washington, D. C. 
2 With regard to the symmetrical interferometer form, cf. Michelson and Morley, Amer. 
J. Sci., New Haven, 31, 1886, (377), also Zeeman, below. 
5 The insufficiency of this equation has been shown by Zeeman, Proc., Amsterdam Acad., 
September 1914, and September 1915 But an estimate only is above in question. 
