PHYSICS: C. BARUS 
135 
rotating rail, whereas m is fixed. The apparatus was also adjustable for 
reversed rays by attaching an auxiliary mirror, normal to the rays d' prolonged 
through n, As S is distant, this slit must be long as otherwise the spectrum 
band will be a mere' horizontal line and the fringes difficult to detect. A 
doublet of lenses each about 10 cm. in diameter, of the same focal power 
(1/60 cm.) but respectively convex and concave and having a combined focal 
distance of about 5 or 6 meters, is of advantage for focussing a large solar 
image (1 to 2 inches in diameter) on the slit, The Nernst or tungsten filament 
gives the same advantages at once, but the former is too thick. 
The fringes are exceedingly difficult to find in spite of the brilliant spectra. 
It was not until after about three days of searching, in which besides sunlight 
I used the filaments as well as the methods of direct and of reversed rays 
that I ultimately succeeded with the former. The fringes lie quite sharply 
in a definite focal plane, usually between that of the slit image and the princi- 
pal focal plane. After being found they are strong elliptic spectrum fringes; 
but when lost nevertheless difficult to rediscover. 
The achromatics which coincide in adjustment with horizontal spectrum 
fringes and are seen with the slit image out of focus, are additionally difficult 
to find because of the short height of the slit image. As first obtained they 
lacked brilliancy and were not easily observed. 
An attempt was made to register slight lateral displacements of the slit 
in terms of the displacement of fringes; but as the slit images are thrown out 
of coincidence when the slit moves, trustworthy numerical data could not be 
obtained. Incidentally it might appear that two vertical lines of the slit 
0.0014 cm. apart should wipe out each other's fringes as a case of interference; 
but this is not the fact as slit widths over 100 times broader are admissible. 
2. After completing these experiments, the distance between slit S and the 
mirrors mn was increased to about nine meters. The same lens doublet 
focussing a large solar image on the slit was used as before. When the 
fringes were found, in view of the longer distance, d, the slit could be opened 
to over a millimeter of breadth before they quite vanished, from the spectrum. 
Operating with two successive slits at about 9 meters from the interferom- 
eter, one of which received the light through the other, I found that two inde- 
pendent sets of fringes very different in size and inclination could be put in 
the field together. Further investigation showed that the size and inclination 
of the fringes is essentially dependent on the degree of parallelism of the two 
corresponding slit images. When the images are parallel, the fringes are of 
maximum size and vertical. When the images are not quite parallel (they 
incline in opposite directions when the slit is slightly rotated in its own plane 
from the vertical), the fringes rapidly grow smaller, rotate and vanish. With 
parallel slit images the spectrum ellipses are centered in the field; otherwise 
they are very far out of center. The adjustment for actual (not x-like) coin- 
cidence must therefore be made with precision, if the fringes are to appear. 
