1893.] 



on The Just-Perceptible Difference. 



23 



out of its middle, over which transparent tracing paper was pasted. A 

 small hole of about J of an inch in diameter was punched out of the 

 centre of the tracing paper ; sixteen minute holes just large enough 

 to allow the entry of the sharp point of a hard lead-pencil were 

 perforated through the tracing paper in a circle round the centre of 

 the hole at a radius of ^ inch. They corresponded to the sixteen princi- 

 pal points of the compass, and had their appropriate letters written 

 by their sides. The outline to be formulated was fixed to a drawing- 

 board, with a T rule laid across it as a guide to the eye in keeping 

 the protractor always parallel to itself. The centre of the small hole 

 was then brought over the beginning of the outline, and a dot was 

 made with the pencil through the perforation nearest to the further 

 course of the outline, and this became the next point of departure. 

 While moving the protractor from the old point to the new one it 

 was stopped on the way, in order that the letter for the bearing 

 might be written through the central hole. These were afterwards 

 copied on a separate piece of paper. 



A clear distinction must be made between the proposed plan and 

 that of recording the angle made by each step from the preceding one. 

 In the latter case, any error of bearing would falsify the direction of 

 all that followed, like a bend in a wire. 



The difficulties of dealing with detached portions of the drawing, 

 such as the eye, were easily surmounted by employing two of the 

 spare letters, R and S, to indicate brackets, and other spare letters to 

 indicate points of reference. The bearings included between an R 

 and an S were taken to signify directive dots, not to be inked in. 

 The points of reference indicated by other letters are those to which 

 the previous bearing leads, and from which the next bearing departs. 

 Here is the formula whence the eye was drawn. It includes a very 

 small part of the profile of the brow, and the directive dots leading 

 thence to the eye. 



The letters should be read from the left to the right, across the 

 vertical lines. They are broken into groups of five, merely for 

 avoiding confusion and for the convenience of after reference. 



The part of the Profile that includes U 

 &c. iiiilU jiihi &c. &c. 



The Eye. 



Letters used as Symbols. 



R....S = (....)• Z=end. 



U, V, T are points of reference. 



By succeeding in so severe a test case as this Greek outline, it 



