17, 6 
613 
Fisher: Low-sun Phenomena in Luzon, III 
like the rim of a fish globe, until about midsunset; then the re- 
maining half-disk begins to show a rim, so that it looks for a time 
not unlike the “tin hat” or trench helmet of the American Expedi- 
tionary Force. This flattens down in the middle faster than it 
shortens horizontally, becomes a line of light, and disappears in 
dots and dashes among the waves, if the horizon is near. The 
disappearance is slow, not like the vanishing of the spot at end 
as in type A. 
I have no definite evidence to explain these two types, my 
attempts to determine refraction not having been successful in 
enough cases to warrant a conclusion. But as type A is always 
associated with horizon mirage, if that is observable, it is doubt- 
less due to the same cause, a reversal of the normal gradient of 
the atmospheric refractive index in the lower layers of the at- 
mosphere. This would explain its peculiarities. 
By analogy, type B would be due to an intensification of the 
normal gradient, in the lower layers of the air. If this were 
great enough, it would cause light to follow the curve of the ocean 
surface, so that objects would be visible at distances indefinitely 
great, or until the light had all gone by absorption. That consid- 
erable intensification of the normal gradient occurs is the reason 
why islands are sometimes visible at unusual distances at sea, as 
observed from the steamer “Venezuela” in June, 1918, when is- 
lands were plainly seen to the north while the ship was in a 
position between Shimonoseki and Quelpart Islands, and nearing 
the latter. One of the navigating officers told me that from that 
or any neighboring position he had never before seen these 
islands in several voyages. Objects on the shore of Martha’s 
Vineyard are sometimes visible over the curve of the horizon 
from Woods Hole which usually are entirely invisible; this is 
generally most marked under the clouds of a retreating north- 
easter. Such a strengthened gradient would cause a strong 
flattening of the sun’s lower limb, so great that it might become 
difficult to judge the true moment of contact, and one would 
probably estimate it in advance of the truth. There would also 
be a flattening of the upper limb as it approached the ocean 
surface, producing a “tin-hat” form, and a final vanishing line 
of light. The observed excessive durations I would, therefore, 
attribute to hasty judgment of the moment of lower contact; 
though with the utmost caution I have been unable to shorten 
these durations. They are certainly not due to carelessness or 
to inexperience in the use of a stopwatch. 
