The Dark Glassesからの英文です。
I started screaming when I got home, and was given a sedative. By evening everyone knew what Miss Simmonds had put the wrong drops in her eyes.
"Will she go blind in that eye, too?" people said.
"The doctor says there's hope."
"There will be an inquiry."
"She was going blind in that eye in any case," they said.
"Ah, but the pain...."
"Whose mistake, hers or his?"
"Joan was there at the time. Joan heard the screams. We had to give her a sedative to calm---"
"--calm her down."
"But who made the mistake?"
"She usually makes up the eye-drops herself. She's got a dispenser's--"
"--dispense's certificate, you know."
"Her name was on the bottle, Joan says."
"Who wrote the name on the bottle? That's the question. They'll find out from the handwriting. If it was Mr Simmonds he'll be disqualified."
"She always wrote the names on the bottles. She'll be put off the dispensers' roll, poor thing."
"They'll lose their licence."
"I got eye-drops from them myself only three weeks ago. If I'd have known what I know now, I'd never have--"
"The doctor says they can't find the bottle, it's got lost."
"No, the sergeant says they've got the bottle. The handwriting is hers. She must have made up the drops herself, poor thing."
"Deadly nightshade, same thing."
"Stuff called atropine. Belladonna. Deadly nightshade."
"It should have been stuff called eserine. That's what she usually had, the doctor says."
"Dr Gray says?"
"Yes, Dr Gray."
Muriel SparkのThe Dark Glassesからの英文です。
主人公（Joan）は15歳ぐらいの女の子です。
Miss SimmondsとMr Simmonds （検眼士）は姉弟です。
主人公が検眼のためSimmondsさんのお店を訪れていたときに、Miss Simmondsが誤った点眼薬を目にさしてしまったところで前回が終わっています。
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最後の方に
"It should have been stuff called eserine. That's what she usually had, the doctor says."
とありますが、
Itはatropineで、Thatはeserineですか？
That's what she usually hadのThatはItとしても意味は通じるような気がするのですが、It should have been stuff called eserine.でItを使っているので、それと分けるためにThatを用いたと考えてよいのでしょうか？
教えてください。お願いします。
（英文が長くなってしまってすみません）
前文は
She had raised the bottle and was reading the label with her one good eye. "Yes, this is mine. It has my name on it," she said.
Dark Basil, dark Dorothy. There was something wrong after all. She walked upstairs with her bottle of eye-drops. The brother put his hand on my elbow and heaved me to my feet, forgetting his coloured slides.
"There's nothing wrong with your eyes. Off you go." He pushed me into the front shop. His flat eyes were wide open as he handed me my glasses. He pointed to the door. "I'm a busy man," he said.
From upstairs came a long scream. Basil jerked open the door for me, but I did not move. Then Dorothy, upstairs, screamed and screamed and screamed. Basil put his hands to his head, covering his eyes. Dorothy appeared on the bend of the stairs, screaming, doubled-up, with both hands covering her good eye.
となっています。
お礼
genmai59さん アドバイスありがとうございます。