Remember, I am not recording the vision of a madman. The sun does not more certainly shine in the heavens than that which I now affirm is true. Some miracle might have produced it, yet the stages of the discovery were distinct and probable. After days and nights of incredible labour and fatigue, I succeeded in discovering the cause of generation and life; nay, more, I became myself capable of bestowing animation upon lifeless matter.(Frankenstein)
I was so far in my reflections when, as I have said, a side light began to shine upon the subject from the laboratory table. I began to perceive more deeply than it has ever yet been stated, the trembling immateriality, the mistlike transience, of this seemingly so solid body in which we walk attired. Certain agents I found to have the power to shake and pluck back that fleshly vestment, even as a wind might toss the curtains of a pavilion. For two good reasons, I will not enter deeply into this scientific branch of my confession. First, because I have been made to learn that the doom and burthen of our life is bound for ever on man's shoulders, and when the attempt is made to cast it off, it but returns upon us with more unfamiliar and more awful pressure. Second, because, as my narrative will make, alas! too evident, my discoveries were incomplete. Enough then, that I not only recognised my natural body from the mere aura and effulgence of certain of the powers that made up my spirit, but managed to compound a drug by which these powers should be dethroned from their supremacy, and a second form and countenance substituted, none the less natural to me because they were the expression, and bore the stamp of lower elements in my soul.(The Strange Case of Dr.Jekyll and Mr.Hyde)
In their different forms we have here the alchemists vision of the transformation of matter, in the one altered materially and spiritually and in the other the elan vital revivified and rescued from corruption in a black resurrection. Hollywood of course took the opportunity to present Hyde as a grotesque monster. It is in some respects an Aristotelian proposal of the soul as the form of the body but when the person is riven into its elements in order that the soul be altered into its shadow the body must reflect that schism.
In the book Hyde is shrunken compared to the ample frame of Jekyll and his face is different but there is no distortion of his features and all that have seen him report a sense of evil writhing beneath the surface.
"He is not easy to describe. There is something wrong with his appearance; something displeasing, something down-right detestable. I never saw a man I so disliked, and yet I scarce know why. He must be deformed somewhere; he gives a strong feeling of deformity, although I couldn't specify the point. He's an extraordinary looking man, and yet I really can name nothing out of the way. No, sir; I can make no hand of it; I can't describe him. And it's not want of memory; for I declare I can see him this moment."
Ted Bundy in an interview on the night before his execution explaining how pornography had made him do it had a similar effect on me and I turned it off as it was late and I didn’t wish to infest my dreams with his attempt to control us right up to the end.
Robert Louis Stevenson had the smooth compression of the essayist and his book runs to a mere 78 pages a novella whose verisimilitude gains from narrators that are doctors and lawyers, sober witnesses though appalled and stricken by the encounter with Hyde. Doctor Lanyon is the only one who has seen the effects of the potion in real time and it shortens his life:
There at least he was not denied admittance; but when he came in, he was shocked at the change which had taken place in the doctor's appearance. He had his death-warrant written legibly upon his face. The rosy man had grown pale; his flesh had fallen away; he was visibly balder and older; and yet it was not so much these tokens of a swift physical decay that arrested the lawyer's notice, as a look in the eye and quality of manner that seemed to testify to some deep-seated terror of the mind. It was unlikely that the doctor should fear death; and yet that was what Utterson was tempted to suspect. "Yes," he thought; "he is a doctor, he must know his own state and that his days are counted; and the knowledge is more than he can bear." And yet when Utterson remarked on his ill-looks, it was with an air of great firmness that Lanyon declared himself a doomed man.
"I have had a shock," he said, "and I shall never recover. It is a question of weeks. Well, life has been pleasant; I liked it; yes, sir, I used to like it. I sometimes think if we knew all, we should be more glad to get away."
Yet when he first saw Hyde he was dressed in he clothes of his larger alter ego and though this would be in the normal way a comic sight nothing about the man encourages this response. In the account of R.L.S. each of us has contending within us the forces of good and evil and his fantasy is being able to give free rein to the dark side without feeling remorse. However when he reverts to Jeykll he feels that objective evil has been wrought and lies out there and he is complicit in it. Resolving to put Hyde back in his hide, a place of watchful evil, and leave him there, his horror at the involuntary eviction of good Jeykll is great. In a reverie after waking he sees that during the night he has been transformed into Hyde and that without the compound.
Now the hand of Henry Jekyll (as you have often remarked) was professional in shape and size: it was large, firm, white and comely. But the hand which I now saw, clearly enough, in the yellow light of a mid-London morning, lying half shut on the bedclothes, was lean, corder, knuckly, of a dusky pallor and thickly shaded with a swart growth of hair. It was the hand of Edward Hyde.,
This is taken from his statement of the history of his doubling to be read after his death. It is a gnostic history of cosmic war fought within the bounds of a single life. Go to the source.