November 6, 19 19] 



NATURE 



253 



The most important invention since Meisen- 

 bach's is the three-colour half-tone process. This 

 was based upon James Clerk Maxwell's researches 

 made so long ago as 1861. The drawing or ob- 

 ject is photographed successively through three 

 colour filters : for the red negative a green filter 

 is used ; for the blue, a red ; and for the yellow, a 

 violet or blue filter. 



A half-tone block is made from each colour 

 negative, an operation requiring the utmost ac- 

 curacy to get register, and the screen is placed at 

 different angles to get white into the interstices 

 of the grain and to prevent an effect like that of 

 "watered silk." 



In all these processes intended for the letterpress 

 machine, the metal plate, for rough work of zinc, 

 and for more delicate work of copper, is mounted 

 "type-high " in the manner described above. 



A more recent invention obviates the use of the 

 objectionable but necessary shiny coated paper : An 

 impression is made from a half-tone plate upon an 

 india-rubber roller and transferred to the paper, 

 which may have an ordinary or even a slightly 

 rough surface. Excellent work has been done 

 with some subjects by the application of this 

 method to the three-colour process, but so far the 

 average results are not equal to those obtained by 

 the use of blocks upon glossy paper. This is 

 called "Off-set." 



A very important photographic process, used 

 until lately more on the Continent than in Eng- 

 land, where it was first introduced in 1870, is 

 collotype ; or, as it was known in earlier days here, 

 "heliotype." Mungo Ponton, in 1839, used 

 bichromate of potassium, and Fox Talbot, in 1851, 

 discovered the action of this chemical in making 

 a gelatine film sensitive to light. When a nega- 

 tive is printed upon a film of gelatine so sensitised, 

 it absorbs moisture in inverse ratio to the amount 

 of light it has received, and when by means of a 

 roller a greasy ink is applied to it, it takes the 

 ink in the ratio of its dryness, and so gradation 

 in the print is obtained. The advantage of this 

 method of reproduction is that it is not necessary 

 to use the glossy coated paper, which is essential 

 if one is to obtain the best result from either a 

 half-tone block in black or from a set printed in 

 three colours. The disadvantage is that it cannot 

 be printed on a letterpress machine in the same 

 way as a block. 



This process is unrivalled for facsimiles qf docu- 

 ments and early manuscripts. But for the repro- 

 duction of pictures and illustrations requiring a 

 greater depth of tone, photogravure remains with- 

 out a rival at present. It is interesting to note that 

 Niepce de Saint-Victor, in 1847, had produced a 

 photogravure plate. He coated a copperplate with 

 bitumen of Judea and exposed it to the action of 

 the sun under a line engraving, which acted as a 

 photographic positive, afterwards biting the pro- 

 tected lines into the copper, and etched a plate 

 which could be printed on a copperplate press. 



Since that time many modifications have been 

 made, the more important being the process 

 invented by Rousillon based upon a beautiful 

 invention of Walter Bentley Woodbury, 

 patented in 1866, and introduced by Messrs. 

 Goupil, of Paris, early in the 'seventies, which 

 was an electrotype from a gelatine mould in 

 relief ; and that by Klic, of Vienna, who in- 

 vented the method now most generally used : A 

 copperplate is covered with an aquatint ground 

 made by dusting powdered resin or bitumen of 

 Judea on it and then melting it with a gentle heat. 

 This causes the particles to run together in little 

 "hills," leaving minute "valleys" between them. 

 Upon this plate an ordinary carbon positive made 

 from a reversed negative is squeegeed down and 

 developed. When it is dry it is placed in a bath 

 of perchloride of iron. This acid bites through 

 the gelatine of the carbon positive and into the 

 copper, the depth being graduated by the varying 

 thickness of the gelatine of the carbon positive. 

 When the biting is completed the gelatine is 

 cleaned off, the copperplate inked by filling the 

 interstices or pits and the excess of ink wiped 

 off, first with canvas and fine muslin, and, finally, 

 with the printer's hand, and an impression taken 

 upon damped paper in the same way as from a 

 copperplate engraved by hand. 



An adaptation of photogravure to machinery 

 was made at Lancaster about twenty years ago. 

 It consists in applying Klic's method to a copper 

 cylinder by the use of a half-tone screen instead of 

 a grain produced by bitumen or resin. After ink- 

 ing the surface of the cylinder it is wiped to re- 

 move the superfluous ink and impressions on paper 

 are made by a rotary motion at a great rate. The 

 process is now largely used for illustrations for 

 weekly illustrated newspapers and magazines. 



PROGRESS IN SCIENCE TEACHING. 



By Sir William A. Tilden, F.R.S. 



A MAN who remembers clearly the first Great 

 •**■ International Exhibition in 1851, and 

 was at school through the period of the Crimean 

 War, can no longer claim to be ranked among 

 young men or even the middle-aged. But, with 

 all the disadvantages of age, there is something 

 to. be said for the satisfaction and practical use 

 of personal reminiscence. The days of school life 

 NO. 2610, VOL. 104] 



which I can recall were practically pre-scientific, 

 for, though one or two schools, such as the 

 Quaker School at Ackworth, included elementary 

 science in their programme, the utmost attempted, 

 as a rule, was a visit from a peripatetic teacher, 

 who came, like the dancing-master and the draw- 

 ing-master, once a week or a fortnight. This 

 was the practice at a school in Norfolk at which 



