On the 12th of October the sun had bidden us good-by for the year, and the rapidly darkening twilight increased the difficulties of the field work.

Our photographs grew daily less satisfactory.

We had not been able to take snapshots since about the middle of September; for, when the sun is near the horizon, though the light is apparently as brilliant as in summer, it seems to have no actinic power.

Our first time-exposures were five seconds; our last, on the 28th of October, were ninety minutes.

The temperature also was gradually getting lower, and on the 29th of October it was 26 deg.