The visibility of a star is a question of brightness simply, and has nothing to do with resolving power.
She applied the brakes and the dust cloud caught up, cloaking the road so thickly that visibility was down to the front of the car.
Disk to depths of 31 and 36 fathoms in the Sargasso Sea, but in the cold currents of the north and also in the equatorial current the depth of visibility was only from zi to 162 fathoms. to the tropical parts of the Indian and the Pacific Oceans the depth of visibility increases again to from 20 to 27 fathoms. Some allowance should be made for the elevation of the sun at the time of observation.
Having this position, the conditions of visibility will be best when the ecliptic, and therefore the axis of the light, are nearly perpendicular to the horizon, and, as the angle between the ecliptic and horizon becomes acute, will deteriorate, slowly at first, more and more rapidly afterwards, owing to the increasing effect of atmospheric absorption.
The low clouds and drizzle made visibility difficult, and the Stroom Bank buoy could not be seen at first.
A transparent atmosphere and clear horizon are necessary, conditions which can best be secured on a mountain top. The visibility of a light corresponding to the inference was shown by Simon Newcomb, by observations at the top of the Brienzer Rothorn, in 1905.
At several stations enjoying a wide prospect the dissipation has been observed to be specially high on days of great visibility when distant mountains can be recognized.
Even if it hadn't been dark, visibility would have been less than a few feet.
Mill has shown that in the North Sea off the Firth of Forth the average depth of visibility of a disk in the winter half-year was 4; fathoms and in the summer half-year 62 fathoms, and, although the greater frequency of rough weather in winter might tend to obscure the effect, individual observations made it plain that the angle of the sun was the main factor in increasing the depth to which the disk remained visible.
In the first place it increased the visibility of the signalling instrument; in the second place it brought that instrument into the position in which it could most readily catch the operator's eye; and finally it eliminated the effort involved in associating one piece of apparatus with another and in finding that other.