336 MODERN PAPER-MAKING
surface areas of paper become detached and adhere to the printing rolls. It is
claimed that this apparatus indicates the relative freedom from 'picking' in
different papers. While the apparatus may give a broad indication as to the
relative merits of different papers, the conditions of the test are not strictly
comparable with those encountered in practice on a printing machine.
An instrument is offered by Schopper to measure the ink-absorption pro-
perties of blotting papers. The same manufacturer is also putting out a con-
tinuous bending tester which it is claimed has overcome some of the fundamental
failures in the design of the folding endurance tester.
During recent years the optical properties of paper have received much
attention, and at the moment quite a number of instruments are marketed
to determine both the opacity and gloss of paper. The use of some of these
instruments is restricted to the routine checking of batches of similar papers
and do not give reliable observations on papers of widely different character-
istics. In the main their operation depends on the measurement of the amount
of light from a standard source passing through the sheet (for opacity) or
reflected from its surface (for gloss) through the medium of a photo-electric
cell. The choice of the photo-electric cell is important, it being essential to
use a cell system with the same colour response as the average human eye.
Amongst the more popular instruments for this purpose are the Westinghouse
Trans-O-Meter, the Bausch and Lomb Opadmeter, and more recently the
Lange reflection meter. A very high sensitivity in the measurement of white-
ness is claimed for the Leukometer (Zeiss).
Composition.—A knowledge of the nature of fibrous materials of which a
sample is composed is of very great importance to the paper-maker. On that
knowledge the price is estimated; without it he may make serious mistakes.
Suppose, for instance, a sample is submitted and a price agreed on to match
it in all respects. The paper offered may be all right in all details, but may
contain a fibre that the customer does not want, and is not in his sample. He
would be quite justified in rejecting the making. It is not at all unusual for
papers containing wood fibres to be offered as matches to 'all-rag' samples
either through ignorance or a desire to make a little more profit, or to get the
order at a 'cut' price.
The microscope is a very necessary piece of apparatus in the identification
of the constituent fibres of die paper furnish, and useful work may be performed
with relatively simple apparatus. We have already (see Chapter I) indicated the
chief points of difference in the various fibres, and little difficulty should be en-
countered in recognizing them. The paper specimen is disintegrated and stained
for examination, the Hersberg stain probably having the most wide application.
Whilst different fibres assume a different tint by the stain, this characteristic