134 MANUAL OF HISTOLOGY. 



quence. Some years before Schafer's theory was advanced 

 Heppner had shown that the bright borders of the black stripe 

 must be due to the reflection of rays through them from the 

 surface of the stripe. The phenomena observed on rotating 

 the stage of the microscope are in accordance with this theory. 

 There are, no doubt, some apparent exceptions, but these lose 

 their weight when we consider how many elements there are 

 that complicate the problem. If, for instance, as is often the 

 case, the disks do not present their edges quite evenly to 

 the eye of the observer, but are somewhat inclined, like the 

 roll of coins above mentioned, the conditions are at once 

 changed. Again, light thrown up vertically through a small 

 diaphragm must produce different effects on the object from 

 light striking it very obliquely. 



Some years ago the writer was fortunate enough to discover 

 an excellent object for the study of living muscular fibre in the 

 detached legs of the Gyrinus. This is a small beetle, known 

 in the country as the "lucky bug," which describes most ec- 

 centric figures on the surface of ponds. A leg should be cut 

 off close to the body, and examined in a drop of water under a 

 very thin covering glass. The shell is transparent, and as the 

 muscles are undisturbed, except in the segment cut in remov- 

 ing the leg, they are in a perfectly normal condition, lacking 

 only their vascular and nervous supply. They will frequently 

 contract for more than an hour, if the covering^ glass be lightly 

 tapped occasionally. The part of the leg known as the tibia, 

 which is easily recognized by a large Y-shaped air-tube, has 

 the thinnest shell, and is usually the best place for study, 

 though occasionally better views are obtained in the two parts 

 next above it, in which it is easier to find a single layer of fibres. 

 The leg is very apt to flex itself between the femur and tibia, 

 thus obscuring one of the best places. If necessary, this can 

 be prevented by putting a thin piece of paper against the inner 

 side of the leg. The anterior pair of legs, which project for- 

 ward, are made on another plan, and are less desirable. A 

 very high power is necessary, as, for instance, Hartnack's 10 

 immersion lens. Much practice also is needed to follow the 

 steps of contraction, and indeed this can be done only when it 

 has become very slow. 



The fibre, when at rest and moderately stretched, appears 

 to be a cylinder with straight edges, and composed of a semi- 



