You might think dark skin would absorb heat and make life miserable in hot countries. But the sun's rays carry more infrared heat than light. Dark skin reflects heat just as white skin does. If it didn't, Nigerians would all have to live in Stockholm. Ben Franklin's pane of glass made a similar point.
Light goes right through glass.
In that sense, glass is opaque. Then he went on to say what his tests suggested about dressing for cold and warm climates. The questions raised by Franklin and Breintnal were not entirely new. They'd undoubtedly read older speculations by Boyle, Boerhaave and Newton on the action of heat and light. Cohen, I. B., Benjamin Franklin's Science.