At the risk of ruining a great friendship, I have to agree with Sam on this debate. The photos I've seen of Titanic's mainmast do not show the fittings necessary for a proper masthead light.
One thing forgotten by those who argue it was an oil lamp is the problem of arc of visibility. Even in 1912 there were strict regulations on this. Note the cutoff shields on Titanic's sidelights for proof. An oil lantern hoisted on a single halyard would be virtually impossible to orient with the correct cutoff angles. And, an all-round oil lamp (360 degree coverage) was not part of the rules.
Unless there was a ladder to carry the lamp up to a hard mounting on the mainmast, I don't see how a proper after masthead oil light could have been displayed. (Oil lamps must be taken down for daily cleaning, wick trimming, and refilling of their reservoirs. They cannot be fixed in position like an electrical light.)
Looking at my copy of ANTR, I find a photo of
Carpathia. The forward masthead light is prominent above the crow's nest. There seems to be a fitting higher on the third mast aft. I've put a glass to it, but the coarse printer screen eliminates any detail.
What were the lights of
Carpathia? Anyone know? My thought is that when people put together their memories they had access to the masthead light or lights of Carpathia, and it was those lights which may have been transferred mistakenly to Titanic. Memory does play tricks. Just a suggestion pending information about Carpathia.
--David G. Brown