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Mr. J. B. Haycraft. Upon the Cause of [Feb. 3, 



bottom of the valleys (D in the diagram), and Hensen's stripe in the 

 centre of the dark band, lies on the exact summit of the ridges. 

 ( H. fig. 1.) 



This position of the stripes in a normal muscular fibre, is the in- 

 variable rule, and the idea at once suggested itself, may not the shape 

 of the fibre itself cause the cross stripings ? 



Any student of natural philosophy would at once affirm that a 

 structureless fibre of such a shape must be cross striped, and a glance 

 at the ribbed neck of the water-bottle on the table will elicit the 

 same answer from any one. 



The question we must now determine is, are the appearances seen in 

 the fibre just the same in all their details, as would be produced by a 

 piece of glass, or any other homogeneous transparent substance of the 

 same shape ? 



Before, however, entering into theoretical grounds, it may be as 

 well to give a full description of what is actually to be seen, for this 

 has yet not been stated. 



With a structure of complicated figure, such as the one we are con- 

 sidering, it is obvious that there is no one focus in which it may be 

 described. There is one pretty definite focus for a single speck or thin 

 film, but even when examining a simple cylinder, it is evident that 

 when the borders of it are clear and distinct, the upper surface is 

 slightly out of focus. We shall see, that in the case of the muscle, 

 although there is one position of the lens when the parts are very 

 distinctly seen, and in which they have mostly been described, yet 

 that on slightly altering the focus, the appearance is changed. These 

 changes we must carefully study. 



For this purpose we may select the large muscles of the thigh of a 

 rabbit; stretch them ever so little upon a piece of wood, and place 

 them for some days in 50 per cent, alcohol. A high power is required 

 for their examination ; I have been in the habit of using a -^-inch 

 of Grundlach, a very perfect lens ; a y^-inch will, however, do. A 

 small bundle of fibrils should be selected in preference to a whole fibre 

 for examination. 



On focussing it becomes at once apparent that on varying the 

 adjustment ever so little, you may bring into focus the tops of the 

 ridges or the bottoms of the valleys which lie between them. Now 

 this slight alteration is sufficient entirely to change the optical appear- 

 ances. 



First raise the lens until the fibre be out of focus and is only to be 

 seen as a dim streak running across the field, then bring it down until 

 its form and the cross markings are distinctly to be seen (the border 

 is now not quite distinct on a level with the horizontal axis of the 

 fibre). In this position alternating light and dark bands are made out, 

 but no vestiges of Hensen's stripes or Dobie's lines. (Fig. 3, a.) The 



