Historical clue formulations

• "Pay attention!" — 2023-10-29
• "Think harder!" — 2023-04-27
• Advice for the easily distracted — 2005-09-30
• Concentrate — 2004-11-15
• Camera concern — 2003-05-27
• Try hard to visualize — 2002-07-10
• Problem-solving advice appropriate for this puzzle? — 2002-06-12
• Think hard — 2000-05-09
• Zero in (on) — 1998-12-20