Complex sentence: The girls said they only went to the beck to see the fairies, and to prove they existed, Elsie borrowed her father's camera, a Midg quarter-plate, and returned 30 minutes later, "triumphant".
Split into simpler sentences: The girls said they only went to the beck to see the fairies. To prove the fairies'existence, Elsie borrowed her father's camera, a Midg quarter-plate, and the girls returned about 30 minutes later, "triumphant".