236 REPORTS ON THE STATE OF SCIENCE. — 1915. 



In some instances the effect of using suitable paper has been spoiled 

 by the use of glaze in the colours or inks. The glossiness of paper is 

 greatly influenced by the extent and particular method of calendering, 

 and it is suggested that careful control of calendering will assist in 

 obtaining the desired hardness and the even surface required, without 

 introducing pernicious gloss. 



Mr. Trotter's description of his gloss-tester is subjoined at the 

 request of the Committee, since the recommendations in this Report 

 require that some standardising instrument should be available. 



4n Instrument for Testing the Gloss on Paper and other Materials. 



The principle of the method is to illuminate a specimen of the 

 paper or other material by light falling on it at an angle of, say, 45 

 degrees. The observer can examine the brightness of the specimen 

 from a direction making an equal and opposite angle of 45 degrees. 

 The effect of the gloss is then a maximum. He can also observe it 

 from a direction nearly parallel with the incident light. The effect of 

 the gloss is then a minimum. The instrument provides means for 

 making these two brightnesses equal, and for comparing them. 



The instrument consists of a box 15 inches (300 mm.) long, 8 inches 

 (203 mm.) wide, and of about the same height. In the bottom there 

 is an opening 3| inches (85 mm.) by If inches (44 mm.). The box may 

 be laid on the page of an open book, and the part of the paper seen 

 through the opening becomes the specimen to be tested. 



A small electric lamp carrying a pointer can be moved on a slide 

 between two mirrors A and B. The lower part of the box is divided by 

 a thin partition. Half of the specimen is illuminated from one mirror 

 and half from the other. Two eye-tubes are arranged at a and b for 

 observing the specimen, the view being obtained alongside the edge of 

 a mirror. In each eye-tube there are a pair of acute-angled prisms 

 edge to edge, by which the view of the thin partition may be cut out 

 of view. 



Let 100 be the total brightness, d the diffused brightness, and s the 

 specular brightness. 100 = rf + s. The pointer attached to the lamp 

 moves over a scale graduated from the formula 



a;=(10-yiOO-s)L/2(10+ v/100^) 



where rr is a length on the scale measured from the middle point, s the 

 specular brightness at 45° expressed as a percentage of the total bright- 

 ness, and L the total length from the middle point of the opening 

 through which the specimen is seen, to one mirror, across to the other 

 mirror, and back to the middle point of the opening. 



The whole instrument is bilaterally symmetrical, and, when the 

 nriirrors are properly adjusted, observations made from either end should 

 give readings equidistant from the middle of the scale. 



When a flattened layer of fine white powder, such as ordinary 

 whiting, is tested, it is found practically free from gloss. In other 

 words, the reflection ir, wholly diffusive, and there is no specular reflec- 

 tion. The brightness of the two halves is identical when the illumina- 

 tion is identical. The pointer is at the middle of the scale when the 



