188 E. A. Schäfer, 



take a different "view of the two appearances from that which has 

 been taken by Rollett. For whereas Rollett, if I rightly comprehend 

 him, looks upon the dark appearance of the longitudinal lines of inter- 

 stitial substance and their enlargements as being due to then* being 

 out of focus, I am of the opinion that they appear thus in the true 

 focus, and that the appearance often seen in deeper focus is illusory. 

 For in the first place, if Ave choose a fibre in which the enlarge- 

 ments of the longitudinal lines are double, the most superficial part 

 of the fibre for a certain depth always has the appearance shown in 

 Fig. 5 A, when it is exactly in focus. (It can be determined when 

 this is the case, by focussing one of the fine ramifications of the 

 tracheae which lie on the surface of the fibre and then sinking the 

 objective ever so little). This is the part of the fibre which can be 

 observed most easily and with least obstruction or interference; all 

 other parts have to be seen through this and their appearance is 

 thereby liable to become modified. Consequently it is probable that 

 the appearance of this superficial part is really the trae appearance of 

 the muscle. 



Secondly, if the fibre is small and well and evenly stretched 

 with the planes of the transverse striae exactly parallel with the axis 

 of the microscope tube, the dark, duplicate appearance of the dots 

 can be seen through the whole thickness of the fibre. But if the 

 planes of the transverse striae are inclined to the axis of the micro- 

 scope, then when the deep parts are focussed an intermediate line of 

 dark dots makes its appearance (Fig. 5 C). This can be produced also 

 in the former case, viz: with exactly parallel planes, if the light be 

 sent from the mirror so as to pass obliquely through these planes. 

 Thus Fig. 21 A and B are two views of the same fibre, taken with- 

 out altering the focus, but in the one case A, with light passing 

 straight between the striae, in the other case B, with light thrown 

 from the mirror obliquely to the planes of the cross-striae; the ap- 

 pearance shown in B, seems therefore to be illusory. This point can 

 be illustrated in an artificial arrangement of parallel pieces of plate 

 glass, fixed at regular intervals from one another. If a beam of light 

 be thrown through such an arrangement parallel to the planes of the 



