1893.] on Tie Just-Perceptible Difference. 25 



a similar outline had been drawn, which had not been pasted together 

 again with perfect precision. The dislocation thereby occasioned 

 would never exceed the thickness of the outline. 



The command of 100 figures from to 99, instead of only 

 26 letters, puts 74 fresh signals at our disposal, which would enable 

 us to use all the 32 points of the compass, instead of 16, and to deal 

 with long lines and curves. I cannot enter into this now, nor into 

 the control of the general accuracy of the picture by means of the 

 distances between the points of triangles each formed by any three 

 points of reference. Neither need I speak of better forms of pro- 

 tractor. There is one on the table by which the ghost of a compass 

 card is thrown on the drawing. It is made of a doubly refracting 

 image of Iceland spar, which throws the so-called " extraordinary " 

 image of the compass card on to the ordinary image of the drawing, 

 and is easy to manipulate. All that I wish now to explain is that 

 this peculiar application of the law of the just-perceptible difference 

 to optical continuity gives us a new power that has practical 

 bearings. 



Postscript. — A promising method for practical purposes that I 

 have tried, is to use " sectional " paper ; that is, paper ruled into very 

 small squares, or else coarse cloth, and either to make the drawing upon 

 it, or else to lay transparent sectional paper or muslin over the drawing. 

 Dots are to be made at distances not exceeding three spaces apart, along 

 the course of the outline, at those intersections of the ruled lines (or 

 threads) that best accord with the outline. Each dot in succession is 

 to be considered as the central point, numbered 44 in the following 



11 21 31 41 51 61 71 



12 22 32 42 52 62 72 



13 23 33 43 53 63 73 



14 24 34 44 54 61 74 



15 25 35 45 55 65 75 



16 26 36 46 56 66 76 



17 27 37 47 57 67 77 



schedule, and the couplet of figures corresponding to the portion of 

 the next dot is to be written with a fine-pointed pencil in the interval 

 between the two dots. These are subsequently copied, and make the 

 formula. By employing 4 for zero, the signs + and — are avoided ; 

 3 standing for —1, 2 for —2, and 1 for —3. The first figure in 

 each couplet defines its horizontal coordinate from zero ; the second 

 figure, its vertical one. Thus any one of 49 different points are indi- 

 cated, corresponding to steps from zero of 0, + 1 3 + 2, and + o 



