The Oval Portraitからの質問です。
Least of all, could it have been that my fancy, shaken from its half slumber, had mistaken the head for that of a living person. I saw at once that the peculiarities of the design, of the vignetting, and of the frame, must have instantly dispelled such idea-- must have prevented even its momentary entertainment. Thinking earnestly upon these points, I remained, for an hour perhaps, half sitting, half reclining, with my vision riveted upon the portrait. At length, satisfied with the true secret of its effect, I fell back within the bed. I had found the spell of the picture in an absolute life-likeliness of expression, which, at first startling, finally confounded, subdued, and appalled me.
Edgar Allan PoeのThe Oval Portraitからの英文なのですが、
Least of all～から始まるこの英文は、"とりわけthat以下（my fancy, shaken from its half slumber, had mistaken the head for that of a living person）でありようにもあり得なかった"、という意味ですか？
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the head for that of a living person
ここのthatはthe headですか？
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its momentary entertainment（瞬間的な余興？）
どういう意味でしょうか？
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satisfied with the true secret of its effect（その効果の本当の秘密に満足して？）
効果の本当の秘密とはどういう事を言っているのでしょうか？
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質問が細かくなってしまってすみません、教えてください。
《前文は、以下です》
The portrait, I have already said, was that of a young girl. It was a mere head and shoulders, done in what is technically termed a vignette manner; much in the style of the favorite heads of Sully. The arms, the bosom, and even the ends of the radiant hair melted imperceptibly into the vague yet deep shadow which formed the back-ground of the whole. The frame was oval, richly gilded and filigreed in Moresque. As a thing of art nothing could be more admirable than the painting itself. But it could have been neither the execution of the work, nor the immortal beauty of the countenance, which had so suddenly and so vehemently moved me.
お礼
ご回答ありがとうございます！ in sooth、in low words、 as of a mighty marvel、のところのご解説、よくわかりました。（in soothというのは目にしたことがなかったのですが、in truth; really という意味なんですね。リンクしてくださったサイトに確かにそうありますね） 質問の個所の英文、とても上手に訳してくださってありがとうございます。