In the first paragraph of “Dr. Heidegger’s Experiment,” the word condescending best describes the narrator’s tone toward the doctor’s visitors. While introducing each of them—Mr. Medbourne, Colonel Killigrew, Mr. Gascoigne, and Widow Wycherly—the narrator displays a superior attitude toward them. He first notes that the men are “venerable” to imply they are worthy of respect. His use of that word, however, is sarcastic. The narrator actually shows little respect for any of the men. The woman is “withered,” or shriveled and sucked dry of vitality. The narrator describes the four friends as

melancholy old creatures, who had been unfortunate in life, and whose greatest misfortune it was that they were not long ago in their graves

In addition to being very elderly, they are portrayed as pathetic and better off dead than alive. The narrator could be younger than they are.

Most importantly, though, he shows himself to be judgmental of each person. In his biased introduction of each character, he editorializes about the person’s past. He presents Mr. Medbourne as a formerly prosperous merchant who “lost his all by a frantic speculation,” ridiculing Medbourne’s investment decisions and portraying him as a senseless gambler. The narrator states that Medbourne is “now little better than a mendicant,” as if the man is a beggar to be looked down upon.

The narrator notes that Colonel Killigrew “wasted his best years” and lost his health and property as a result of “pursuit of sinful pleasures.”

He hints at Mr. Gascoigne’s disreputable past as a

a man of evil fame, or at least had been so till time had buried him from the knowledge of the present generation, and made him obscure instead of infamous.

The narrator suggests that Gascoigne is lucky that the cause of his infamy is forgotten. The man is so old that people today no longer know what he did. Thus, the ignominy of this “ruined politician” is safely “buried” or concealed by passing years.

The lone woman is Widow Wycherly, who was supposedly “a great beauty in her day.” Now she lived in seclusion from society due to shame from “certain scandalous stories which had prejudiced the gentry of the town against her.” The narrator may or may not be of a higher social status than her. Either he speaks haughtily of this fallen woman or he ridicules the social hierarchy of the “gentry” that cast out Wycherly. In any case, his note that she was plagued by “scandalous stories” implies she was loose and consorted with many men.

The narrator then makes the point that

It is a circumstance worth mentioning that each of these three old gentlemen, Mr. Medbourne, Colonel Killigrew, and Mr. Gascoigne, were early lovers of the Widow Wycherly, and had once been on the point of cutting each other's throats for her sake.

The phrase “It is a circumstance worth mentioning” seems like an attention-seeking aside like “Oh, by the way.” The narrator emphasizes that these three men all fell for Widow Wycherly and were so smitten with her that they were ready to fight to the death over her.

Finally, before telling the action, the narrator states

And, before proceeding further, I will merely hint that Dr. Heidegger and all his four guests were sometimes thought to be a little beside themselves—as is not unfrequently the case with old people, when worried either by present troubles or woeful recollections.

The phrase “before proceeding further” is like the narrator is clearing his throat and highlighting the importance of what he is about to say. He will “merely hint” that the characters “were sometimes thought to be” unstable; he points out that fact but will not take any responsibility for it.

In fact, by using the passive voice and thus attributing to this claim to others, the narrator is able to mock the elderly for being “a little beside themselves” without eliciting personal blame. He emphasizes that the four are so affected by strong emotion that they do not always have control over themselves. The statement “as is not unfrequently the case with old people” is the narrator’s way of saying, “Oh, you know how these geezers can be.” By noting “woeful recollections,” the narrator does not let the characters (or reader) forget their individual and collective embarrassing past histories.

There are a number of words I think you can choose from to describe the narrator's tone towards Dr. Heidegger's guests in the opening paragraph. The tone is at once dismissive, contemptuous, condescending and conspiratorial.

In the opening paragraph, the narrator describes Dr. Heidegger's visitors as, "all melancholy old creatures." He continues to suggest that the greatest misfortune of these visitors was "that they were not long ago in their graves." The tone here is rather contemptuous and dismissive. The word "creatures," for example, suggests that the narrator can't quite bring himself to acknowledge that they are humans, or at least civilized humans. Describing their greatest misfortune as the fact that they were not yet dead is a little cruel, and hyperbolic, emphasizing the narrator's contempt for the visitors.

Towards the end of the first paragraph, the narrator says, "before proceeding further, I will merely hint that Dr. Heidegger and all his foul guests were sometimes thought to be a little beside themselves." The narrator's contempt for the visitors is again clear here. Indeed, he calls them "foul guests." The phrase, "a little beside themselves," is euphemistic, and also rather condescending, or patronizing. It's as if the narrator is speaking about very small children, or rather mad, very elderly people. When the narrator says, "before proceeding further, I will merely hint," the tone is somewhat conspiratorial, as if the narrator is letting us, the readers, in on a secret. It feels at this point as if the narrator is speaking confidentially to us, while the guests carry on unawares in the background. This is a way in which the narrator can make us side with him, against the guests.