968 SUMMARY OF CURRENT RESEARCHES RELATING TO 



appear darker. Alter the focus by screwing tlie lens up or down, 

 and, provided the fibre can still be seen, this state of matters will be 

 reversed ; for after converging, the rays above the position of the 

 ridges will now be diverging, while at the same time those over the 

 valleys will be converging and will appear bright. 



" The condition which is intermediate between the low and high 

 focussed picture of the fibre, would be obtained by shifting the lens 

 half-way between these two positions. Hensen's stripe is no doubt 

 due to rays passing through the centre of the ridges suffering little 

 refraction in their course, and thus causing a l)rightness. Dobie's 

 line might, of course, be the reverse of this, no rays at this point 

 coming to the eye of the observer ; but we shall speak of this more 

 hereafter, when we shall show that there is some reason for sus- 

 pecting at this point a distinct structure. 



" Although it is indispensable to account theoretically for these 

 appearances, yet to most persons a simple demonstration will carry 

 more conviction than any proof deduced from the laws of optics, 

 however well they be understood. Instead of showing ' what should 

 be,' we will study ' what is.' 



" For this purpose we will imitate as nearly as possible the figure 

 of a muscular fibre on a small scale, and it shall be made out of a 

 substance of uniform consistence throughout. What appearances 

 will it present on microscopic examination ? I have proceeded in 

 the following manner : — A glass rod is heated in a spirit lamp and 

 plunged into a bottle of Canada balsam ; it is then withdrawn, and 

 a little drop of the balsam is allowed to fail on a glass slide, or a 

 thread of it may be laid out on the surface of the glass. Before the 

 drop or thread has solidified it is indented with the milled head of 

 a fine screw, and examined with a power of from twenty to fifty 

 diameters, when cross shadings are to be observed. These are seen, 

 moreover, to correspond with the surface impressions, and not only 

 so, but they are reversed on altering the focus. Hensen's stripe is 

 generally very well seen. The most beautiful and convincing object 

 to study in this connection is a scale of the Lepisma. These are sold 

 as test objects with many Microscopes. They are oval in shape, 

 transparent, and singly refractile throughout, and beautifully ribbed 

 in their length, these ribbings or groovings being indeed so tine that 

 a power of at least 500 diameters will be required to make out those 

 points to be here described. You would think on looking at one of 

 these scales that a piece of muscle was flattened out before you on 

 the field : no rough balsam model, but a perfect illustration taken 

 from the back of a tiny insect. 



" The appearances it is needless to describe, for they are, almost 

 to the minutest detail, those of a muscular fibre. The bright and 

 dark stripe interchanging with every alteration of focus, Hensen's 

 stripe and Dobie's line (Krause's membrane) are all to be seen. 

 In the case of Lepisma scale the line of Dobie is in the centre of a 

 bright band, which is broader than the dark band with Hensen's 

 stripe. This is, of course, the other way in the case of the muscular 

 fibre. 



