Chapter 1: Section 7: Page 64: Line 153 (1135)

 Like anybody, of course, he had wondered what happened during the mysteriously guarded transition from plate to print, but never enough to step across any darkroom's forbidden doorsill to have a look.

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Pynchon has a lot going on in this sentence but first and foremost is his alluding to transubstantiation except in reverse: instead of an object becoming flesh, flesh, in photography, is transformed into an object. He couches it in the "mysteriously guarded transition from plate to print" but that's just allowing Merle to ease into the subject of flesh to print. In the span of two lines, Merle has expanded his idea of photography from an idiot's game to a religious concept.

"mysteriously guarded transition"
Merle will step back slightly from this characterization in the next half of the sentence when he admits that he's in control of where and when he could learn this secret but here, at the start, he sounds like he's a subscriber to the Proverbs for Paranoids. Something guarded must have guards. Rather, Guards, capitalized. As in They. Them. The Masters controlling the information.

"but never enough"
People often become obstacles to sating their own curiosity (and that's if they can even bother to find themselves curious about the nature of things at all). Ignorance comes in many forms but the withholding of information from yourself is the foundation of one of the most common. The most common probably stems from people simply not understanding complex answers to questions and seeking the safety of a simple solution. These are the people who believe they're smarter than the most intelligent people because they have common sense. Common sense feels like intelligence but it's actually just a smokescreen to understanding. Common sense says the sun revolves around the Earth because that's a common and easily digested common observation. But what common sense often just hides is ignorance based on a lack of understanding a complex world where observation isn't the only answer.
    Before you think I've gone off on a tangent, let's observe this: photography becomes an easy allusion to discuss light and vision, a comparative to human perception and observation.

"darkroom's forbidden doorsill"
Pynchon uses "doorsill" purposefully since it's a synonym for threshold, or the point of beginning. The term "darkroom" indicating ignorance and the lack of knowledge, forbidden because the knowledge remains particular to those who study photography and their apprentices. With Roswell's loss of apprentice, Merle has filled the vacuum to be allowed this knowledge. He steps over the threshold to begin his new life as a photographer, a capturer of moments, one who sees and observes particularly well.

"have a look"
Once again, it's observing that's important, with the basic understanding that light is essential for "looking". Perhaps this encounter with photography and the need to understand how light can be used to save images in time and space was needed for Merle to encounter and fall in love with Erlys, a name that means "the light" in Norwegian. Remember, this is the story about how Merle fell in love with Erlys Mills!

Chapter 1: Section 7: Page 64: Line 152 (1134)

 It had always seemed like an idiot's game, line them up, squeeze the bulb, take the money.

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"an idiot's game"
Weirdly, I could find no etymology on this phrase. Yes, I only used various search engines on the Internet and not hiring a bunch of research assistants or visiting the local library. Partly this is because the Internet is broken and gives weight to popularity of use. So instead of discovering why this phrase seems to have some weight as an actual idiom, I'm overwhelmed by pages and pages of people quoting some character in Red Dead Redemption 2. Being that the phrase is just two common words stuck together which connote an easily digestible idea, the phrase isn't particularly looked upon as an idiom, even if I saw examples of it used in so many diverse spaces that it feels like a common saying.
    Everybody reading this phrase gets the point so I guess it doesn't need an etymological breakdown in the Oxford English Dictionary. Merle's describing a game that only idiots would play, like Three Card Monty or Russian Roulette. He's hinting at the simplicity of people who fall for fads like this. It's his initial reaction to the entire concept of photography, looking down his nose at a budding new technology that seems to serve no purpose other than to bolster the vanity of those paying for their pictures.

"line them up, squeeze the bulb, take the money"
Wait. Is prostitution also an idiot's game?!
    Anyway, Merle first thought of photography as a scam to take people's cash.

Chapter 1: Section 7: Page 64: Line 151 (1133)

 Merle was no hoosier on the subject, he had seen cameras before, even had himself snapped once or twice.

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"no hoosier on the subject"
Pynchon uses the term here to denote a "yokel" and not a "resident of Indiana". This makes sense in context as the rest of the sentence offers proof to how Merle isn't completely inexperienced with the idea of photography.
    I mean, not a lot of evidence! Merle has seen some cameras and had his picture taken. I'm not sure I'd allow that as evidence as to Merle's expertise on the subject. I suppose, this taking place in the 1880s, just seeing a camera and posing for a photo once or twice makes you leaps and bounds more proficient on the subject than the average American hoosier on the frontier.

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 Since Roswell had only been in the asylum for a day or two, they found his equipment untouched by local scavengers or the landlord.

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Pynchon often shows his contempt for the police. I'm glad to see he treats landlords with the same disrespect. He has only mentioned landlords once before but in a similar context. Landlords hold an inordinate amount of power over people's lives and so they can, on a whim, evict a tenant or, as hinted at here, simply steal from a tenant who hasn't been around for a bit. The suggestion here might be that Roswell pays rent by the week and so just a few days didn't alert the landlord to his absence. But it also suggests that if Roswell had been incarcerated for a little longer, the landlord would have quickly swooped in and claimed his possessions, selling them off to pay for the missed rent. Or even that the landlord simply would have stolen from him if he thought he could, rent paid or not.

The mention of local scavengers points to two things: the general poverty level of the vacancies the visiting Ætherists have taken up and as a direct comparative to landlords. And the term, "landlord", of course, is a term rife with imperialist and capitalist symbolism, indicating that they are one of the bad guys of the novel.

Chapter 1: Section 7: Page 63: Lines 148-149 (1130-1131)

 "Better than that, my apprentice ran off when the coppers showed up. How'd you like to learn the deepest secrets of the photographer's trade?"

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Not only was Roswell just rescued from Hell, it's possible he was actually a devil too? Here he's offering Merle a Faustian deal to learn the "deepest secrets" of his vocation. You don't offer somebody a job by offering to teach them not just deep secrets but the deepest secrets. This sounds like a deal with the devil more than an offer of apprenticeship.

We still haven't gotten to the point of the story (Merle falling in love with Erlys) but at least we've now gotten the secret origin of Merle becoming a photographer. And that's probably an important part of how he met Erlys being that we know Merle loves to photograph naked ladies. Wait. Is that the deepest secret of photography? Women will readily take their clothes off so you can take sexy pictures of them? The amount of naked women I've seen based on my writing that doesn't include taking breaks to look at image searches of weird Rule 34 stuff pales in comparison. Maybe I'll drop the writing and pick up a camera? Or re-pick up a camera since I still have that fancy camera that I bought for my cross-country VW bus roadtrip off  of an office cubicle installer when I was managing the Netscape Warehouse back in the '90s (which include a huge bag of funny mushrooms thrown in the case because he loved me so much!). Knowing my luck, I'm already 135 years too late for women casting their clothes aside simply because a man points a newfangled camera in front of them. Stupid me being born so late into the 20th Century!

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 "Buy the next round, we'll call it even."

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Roswell Bounce purchases redemption easily enough. He goes from sinner to righteous with the purchase of a round of whiskeys. From Preterite to Predestined, as Pynchon might say.

Yeah, yeah. I know you can't go from Preterite to Predestined, theologically! The whole point is that each is already decided before you were even born! But, being that human beings are incapable of attaining that knowledge in life, they can still go from believing they're saved or lost from any moment to the next. Obviously they'll never actually know until they die. Although, they won't really know then either because they'll be, you know, dead. Non-existent. Back to the material from which they came. Deletion of the ego. The end. Kaput. That's all she wrote, folks.

Chapter 1: Section 7: Page 63: Lines 145-146 (1127-1128)

 "I could've been in the chapel at that dance where the fire broke out. Guess you saved my fundament, there."

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"the chapel at that dance where the fire broke out"
The Northern Ohio Insane Asylum has been painted as Hell and the fire which broke out began in the one possibly holy place in the entire complex. A chaotic reaction of evil against the holy trying to purify it.
    Roswell points out that he "could've" been in the chapel but wasn't. Probably because he's cynical. He's merely reacting as anybody might react when they've fled a dangerous situation: "It could have been worse. I could have died. I was just lucky enough to be where I was in the midst of the danger." But we all know Roswell never actually would have been in the chapel.

"saved my fundament"
Literally, "You saved my ass." But the word "fundament" is probably meant to make the reader think of fundamentalist religions, just to keep the "Insane Asylum as Hell" and "Saloon as Church" analogy going. Merle not only saved his ass but the continuation of his belief system.