_The Camera Lucida._--I should feel much obliged to any reader of "N. & Q."
who would be kind enough to answer the following questions, and refer me to
any work treating of the handling and management of the Camera Lucida. I
have one made by King of Bristol, and purchased about thirty years ago: it
draws out, like a telescope, in three pieces, each six inches long; and at
full length will give a picture of the dimensions of twenty inches by
twelve. The upper piece is marked from above downwards, thus: at two inches
below the lens, "2;" at an inch below that point, "3;" at half an inch
lower, "4;" at half an inch lower still, "5;" half an inch below the point
"5," a "7" is marked; and half an inch below the "7," there is a "10;" at
seven-eighths below this last, "D" is marked. What reference have these
nicely graduated points to the distance of an object from the instrument?
Do the figures merely determine the size of the picture to be taken? How is
one to be guided in their use and application to practice?