"That is certainly a very pretty deception,'' said the doctor's friends; carelessly, however, for they had witnessed greater miracles at a conjurer's show; "pray how was it effected?'' (12)
The guests' skepticism reminds the reader that such transformations could be tricks of the eye; we then have to doubt the reality of the next transformation we see (that of the guests themselves).
The liquor, if it really possessed such virtues as Dr. Heidegger imputed to it, could not have been bestowed on four human beings who needed it more woefully. (22)
The narrator keeps the nature of the guests' transformation ambiguous with lines like this one. We don't know for sure if the liquor really does possess the virtues that Dr. Heidegger claims, and the narrator won't tell us one way or the other.
Assuredly there was an almost immediate improvement in the aspect of the party, not unlike what might have been produced by a glass of generous wine, together with a sudden glow of cheerful sunshine brightening over all their visages at once.
This line – along with the description of the bubbling water – suggests that the liquid might actually be alcohol. This is an explanation for the evening's events which keeps the story out of the realm of the supernatural.
There, in fact, stood the four glasses, brimful of this wonderful water, the delicate spray of which, as it effervesced from the surface, resembled the tremulous glitter of diamonds. It was now so nearly sunset that the chamber had grown duskier than ever; but a mild and moonlike splendor gleamed from within the vase, and rested alike on the four guests and on the doctor's venerable figure. (32)
It's probably not a coincidence that, as the evening's events transpire, the sun sets. There is a much bigger transformation in the setting which accompanies the transformation taking place inside the Doctor's study.
He sat in a high-backed, elaborately-carved, oaken arm-chair, with a gray dignity of aspect that might have well befitted that very Father Time, whose power had never been disputed, save by this fortunate company. Even while quaffing the third draught of the Fountain of Youth, they were almost awed by the expression of his mysterious visage. (32)
Dr. Heidegger, too, has been transformed in the course of the evening. His "visage" has certainly changed, though whether this change is manifested only in the eyes of his guests is subject to debate.
The fresh gloss of the soul, so early lost, and without which the world's successive scenes had been but a gallery of faded pictures, again threw its enchantment over all their prospects. They felt like new-created beings in a new-created universe. (33)
Lines like this one support the theory that Dr. Heidegger is a godly figure. If the guests are newly-created beings, then it is Dr. Heidegger who has created them. If the world is a newly-created world, then, again, Dr. Heidegger is its creator.
Blushing, panting, struggling, chiding, laughing, her warm breath fanning each of their faces by turns, she strove to disengage herself, yet still remained in their triple embrace. Never was there a livelier picture of youthful rivalship, with bewitching beauty for the prize. Yet, by a strange deception, owing to the duskiness of the chamber, and the antique dresses which they still wore, the tall mirror is said to have reflected the figures of the three old, gray, withered grandsires, ridiculously contending for the skinny ugliness of a shriveled grandam. (41)
Does this tell us anything about the nature of the guests' transformation? Or does it leave us in the dark as to what's really happening here? Does it simplify, or further complicate, our reading of the narrative?
His guests shivered again. A strange chillness, whether of the body or spirit they could not tell, was creeping gradually over them all. (48)
This line raises an interesting question: what is the difference between a transformation of the body and a transformation of the spirit? How does the story comment on this divide?