224 



THE MICROSCOPE AND ITS REVELATIONS. 



scale) is raised, either by corrugation or thickening, into a series of 

 strongly- marked longitudinal ribs, which run nearly parallel from one 

 end of the scale to the other, and are particularly distinct at its margins 

 and at its free extremity; whilst the other surface (the free or outer, 

 according to Mr. J. Beck) presents a set of less definite corrugations, 

 radiating from the pedicle, where they are strongest, towards the sides 

 and free extremity of the scale, and therefore crossing the parallel ribs at 

 angles more or less acute (Fig. 417). It was further pointed out by Mr. 

 K. Beck, that the intersection of these two sets of corrugations at differ- 

 ent angles produces most curious effects upon the appearances which 

 optically represent them. For where the diverging ribs cross the longi- 

 tudinal ribs very obliquely, as they do near the free extremity of the 



FIG. 417. 



FIG. 418. 



Scale of Lepisma, saccharina. 



[Scale of Machilis polypoda. 



scale, the longitudinal ribs seem broken up into a series of ' exclamation- 

 markings,' like those of the Podura; but where the crossing is transverse 

 or nearly so, as at the sides of the scale, an appearance is presented as of 

 successions of large bright beads. The conclusion drawn by the Messrs. 

 Beck, that these interrupted appearances are "produced by two sets of 

 uninterrupted lines on different surfaces," has been confirmed by the 

 careful investigations of Mr. Morehouse. 1 The minute beaded structure 

 observed by Dr. Koyston-Pigott 2 alike in the ribs and in the intervening 

 spaces, may now be pretty certainly regarded as an optical effect of dif- 

 fraction ( 156). In the scale of a type nearly allied to Lepisma, the 

 Machilis polypoda, the very distinct ribbing (Fig. 418) is produced by the 



1 "Monthly Microsc. Journal," Vol. xi. (1874), p. 13, and Vol. xviii. (1877), 

 p. 81. 



2 "Monthly Microsc. Journ.," Vol. ix. (1873), p. 63. 



