378 



Transactions of the Society. 



nation — "blue" and "red" are, of course, used in this connection 

 vaguely and inaccurately to express complementary colours generally 

 — hence the flanking lines are, like the principal, white lines, but 

 with the addition of coloured extremities where the mixture of com- 

 plementary colours is incomplete. Having thus obtained a graphic 

 expression for the short edge of the parallelogram, we may proceed 

 to develop the whole parallelogram from it as before, with the result 

 shown in fig. 85. It is obvious that by giving a different angular 



position to the axis of the an ti point, 

 Fig. 85. we might have produced correspond- 



ingly modified images in which the 

 flanking members would have stood 

 at varying distances from the prin- 

 cipal image, and both principal and 



Haukinfi- 



mages 



would have varied 



slightly in breadth. This \% ill be 

 easily appreciated with the help of 

 fig. 83. 



We are now in a position to 

 understand the phenomena presented 

 to our observation w-hen the ' 01 in. 

 ruled surface is viewed through the 

 ' 0025 in. grating. As the grating 

 is rotated over the field of the Micro- 

 scope, the object takes on a series of 

 appearances represented in some of 

 their more marked phases by fig. 86. 

 The drawing is self-explanatory, but 

 a word or two may be bestowed upon 

 some of its characteristic features 

 Thus it is to be observed that the 

 phase marked I. in the drawing, 

 although representing the object 

 with considerable precision, does not 

 present an absolutely true picture, for 

 the lines, although not repeated iu 

 a lateral direction, are repeated in 

 a longitudinal direction, and hence are exaggerated in length, and 

 terminate, not in clear-cut extremities, but in graduated ends. Again, 

 the phase marked II. gives rise to a trebling of the number of lines, 

 and to a serrated appearance along the upper and lower margins of 

 the figure. The phase marked III. causes the number of lines to be 

 doubled, the fictitious lines being feebler than their principals, but in 

 the central portion so strengthened by reinforcement where they over- 

 lie one another, that they appear of equal, or almost equal, brightness 

 with the real lines. 



These appearances are all easily and directly explained by reference 



