370 



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



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 appear- 

 ances, 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 a bottle of Canada balsam ; it is then withdrawn, and a little drop of 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 fall 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 connexion 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 fine 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 

 Dcbie's line (Krause's membrane) are all to be seen. In the case of 

 the 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. 



We see, therefore, that a muscular fibre presents just those appear- 

 ances which a transparent body of uniform texture and of similar 

 shape would possess. However conclusive these proofs may have 

 been, it is well to collect all evidence possible to show that these 

 markings are nothing more than optical effects, to which end a very 

 searching experiment was suggested to me by Professor Tait. It is 

 evident that if these cross bands are seen when parallel, or nearly 



