Page:Paper and Its Uses.djvu/113

Rh papers to be compared brought edge to edge over the printed matter, and the comparative degree of opacity observed by the ease, or otherwise, with which the lettering can be seen through the papers.

Surface or Finish.—The degree of polish which is given to the paper may be compared by feeling with the hand, and also by looking along the sheet. The comparisons can be brought to a numerical basis, but apparatus which would seldom be used is required for that purpose.

Resistance to Wear.—An ingenious machine is made for testing the resistance which paper offers to wear as exemplified by repeated folding. A narrow strip is clamped at each end, kept under constant tension by springs, and folded backwards and forwards until breakage occurs. Some idea of the comparison can be seen by taking the following results:—

The tests by the folding machine, being conducted on a very narrow strip, are only moderately reliable.