ee ee ne 
W. H. Pulsifer—Thickness of Young's Reversing Layer. 303 
Art. XXXVII—On a Method of Estimating the Thickness of 
Young’s Reversing Layer ; by W. H. Putstrer. 
A part of my duties, asa member of the Fort Worth Eclipse 
party, consisted of the spectroscopic observation of the con- 
tacts. I used a telespectroscope combining a four-inch Clark 
telescope of four feet ten inches focal length, and a ten-prism 
Browning solar spectroscope. The field included the lines 
between W. L. 6600 and 6300. 
At second contact I observed the reversal of the Fraunhofer 
lines, and was surprised to find the reversed lines shortened at 
each end, and occupying but about one-third of the width of 
the spectrum, while the C line was not shortened and remained 
in view after the other lines had disappeared. At the moment 
no explanation of the phenomenon presented itself, but after- 
ward it occurred to me that it was occasioned by the extension 
of the slit of my spectroscope beyond the reversion layer on 
each side, and that a measurement of the image of the sun 
formed on the slit, and of the length of the slit itself, would 
enable me to estimate the thickness of the reversing layer. 
Careful measurement showed the diameter of the sun's image 
on the slit to be 0°54, and the length of the slit 0-08". : 
_ In the diagram, C represents the chromosphere, R the revers- 
ing layer, S the slit, a b ¢ the shortened lines, and dé the 
radius of the sun. The difference in the length’ of the lines 
dband de represents the minimum thickness of the layer, as 
Indicated by the reversed lines observed. Assuming the sun's 
diameter to be 860,000 miles, and accepting the measurement 
e sun’s image and of the slit, and the statement that the 
reversed lines occupied but one-third of the width of the spec- 
trum, the difference in the length of the lines d 6 anddcis 
found to be 524 miles. 
