Chapter 1: Section 7: Page 65: Line 191 (1173)

 He parked the wagon out on a vacant patch in Murray Hill and set in to study the mysteries of light-portraiture as then understood, gathering information dip-fingered and without shame from everyplace he could, from Roswell Bounce to the Cleveland Library, which as Merle soon discovered had taken the revolutionary step some ten years before of opening up its stacks, so anybody could walk in and spend all day reading what they needed to know for whatever it was they had in mind to do.

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"He parked the wagon out on a vacant patch in Murray Hill"
Merle's isolating because what he's up to often gets one thrown into the local nuthatchery. The people the cops currently hate most in Cleveland are people who disrespect them, people who witness them doing crime, people super interested in science and light, loiterers, and foreigners. Merle fits into most of those categories so he needs to be careful. Especially since he's the guy who breaks everybody out of Newburgh. If he gets picked up, who's going to rescue him?

"study the mysteries of light-portraiture as then understood"
The year was 1887 or thereabouts so if you want to know exactly how well "light-portraiture" was understood, get thee to a set of Encyclopedia Britannica, I guess. Or just the L Volume? What was the last year Encyclopedia Britannica published an actual update to their set? How out of date must the final version of their encyclopedias be at this time? And how terrible are they online now? I bet they've incorporated AI into their site so you can't trust a thing it says. The thing about LLMs and AI as they're currently trained is they aggregate information from online sources. And a lot of them just scour any old thing. So you've got to assume the smartest an "AI" can be is of median intelligence. And anybody who's spent any time with more than themselves knows that other people are stupid idiots.
    Wait, what was I talking about? I suppose I could look up all that information on Encyclopedia Britannica but it sounds like boring research. It's not like I have a report due on the exploration of space.

"gathering information dip-fingered"
Stealing when he must! Although who's going to have information about light-portraiture in the area? Is Pynchon suggesting that Merle was ripping of Roswell Bounce?! Oh, yeah, he goes on to say that exactly. From Roswell and the Library!

"the Cleveland Library"
Merle states the Cleveland Library had just opened its stacks in the late 1870s and being that Pynchon wrote it, I'll take his word for it. Maybe Merle just doesn't know the local history. Maybe by "opening up its stacks", it means something more specific than just opening to the public and letting them wander around looking at the books on loan. Apparently public libraries were a fairly new idea for the 19th century! Before that, it was all private libraries and colleges and weird uncles driven to madness by strange, flesh-covered books from the Middle East.

"taken the revolutionary step some ten years before of opening up its stacks, so anybody could walk in and spend all day reading what they needed to know for whatever it was they had in mind to do"
Did Pynchon accept money from the Public Library lobby to insert this into his book? Sounds like he's trying to sell me on libraries! Ew!

Chapter 1: Section 7: Page 65: Line 190 (1172)

 Merle's all-night illumination prolonged itself into an inescapable glow that began to keep him awake.

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Recently, I read something by somebody about Thomas Pynchon saying that his individual sentences and written scenes were easily understood but it was the larger themes which caused all the problems. But I've just read this sentence about eighteen thousand times and I don't know what Pynchon is trying to tell me. Is it that Merle's obsession with learning about photography and light have begun to keep him up at all hours, being that "illumination" is a synonym for "knowledge" (as has been discussed by me ad nauseum)? Hmm, now that I ask that question, I think that's exactly what Pynchon is saying and using light terminology because it's a cute way to phrase it. Pynchon could have added a footnote to this line that just read, "Look how clever I am, you dumb illiterate douchebag! Ha ha! I'm smart! You're stupid! Suck it (and by 'it', I mean my penis, natch!)!"

"Merle's all-night illumination"
A metaphor, I suspect, for the idea of "cramming" for a big test with the added idea that Merle was burning a candle or lantern all night so that he could read, study, and experiment.

"prolonged itself into an inescapable glow"
The "prolonged" here is modifying the initial "all-night" meaning that Merle's cram session soon extended into the following days and weeks. He was unable to escape his need to learn more about light. The "inescapable glow" perhaps suggesting the will-o-the-wisp as metaphor for the knowledge of light Merle now obsessively pursues.

"that began to keep him awake"
Awake because he's busy studying and experimenting but also "began to keep him awake" because he's become obsessed with the subject. He has found his hobbyhorse and he can't stop himself from riding.

Chapter 1: Section 7: Pages 64-65: Lines 188-189 (1170-1171)

 "Just what people have been noticing ever since the first sunburn," Roswell shrugged, "which is that light makes things change color. The professors call it 'photochemistry.'"

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I adore this bit from Roswell Bounce. Some writers might choose to make Roswell an expert on all things photography rather than just a kind of ignorant conspiracy theorist who understands the idea of how science gets to where it is by building upon the perceptions and discoveries of those who have come before while also being completely wrong about his facts. The idea that sunburn was noticed as light being able to make things change color is obviously laughable. It's like discovering that the first property of fire mankind noticed, which led them to experiment with it, was how it changes meat from pink to brown and black. "Oh hey! This fire over here makes things change color! Maybe we can someday use it in photography!" Sure, Roswell. A sunburn causes a Caucasian arm to turn pink or red. But nobody thought, "Hey! Light makes things change color! I've just discovered photochemistry!" Or, should I be more specific, and say, "Some really dumb scientists made really stupid initial conclusions but after a couple of experiments where half of them died of skin cancer, they realized the whole 'color changing bit' was a red herring!"

Look, maybe I'm wrong! Maybe people did assume light caused sunburns because it had color changing properties! I'm a modern man living in the 21st century and I'm incapable of believing that even men of the 19th Century believed stupid nonsense! But I trust Pynchon's just deep in the character of Roswell Bounce here, acting the expert when really he's just a regular guy who knows how to process photographs. Why should he care about the history of it all? He's just trying to sound smart to his new apprentice! We all know that sunburns aren't caused by light's ability to change the color of things! It's because of the tiny homunculi riding the particles along the UV rays that dig into your flesh like alien spirochetes. Duh!

"photochemistry"
At the time Roswell makes this statement, the scientific world only had one law of Photochemistry: light makes things change color. No, no! That was a joke! Ha ha! The law was Grotthuss-Draper Law, first proposed by Grotthuss in 1817 and later by Draper in 1842, totally independently and not at all in a plagiarized manner. I mean, how the hell do you get partial credit for coming up with a scientific law 25 years after somebody else came up with it? I'm sure he added some cool spin to it or something.
    The Grotthuss-Draper Law posits that systems can only be changed by light if the light is absorbed into the system. I guess in 1817 (and 1842!), that was a revelatory statement. People were probably trying to tan their mirrors and Grotthuss was all, "No, no! Mirrors don't absorb light, you idiot! You need, like, a plant or a person or, um, water? I don't know! Wait until 1842 and it'll all get worked out a little better!"