ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 133 



existing in the object when its character is not altered by an inclination 

 of the incident pencil of light (oblique illumination). If the character 

 is altered in passing from central to oblique illumination, we may con- 

 clude that in the latter, diffracted rays enter the Microscope which were 

 unable to do so in the first case. When this happens, it indicates that 

 a complete representation is not obtained by central illumination, and 

 it must be doubtful whether it is so by oblique illumination. 



We may obtain a good idea of the optical processes which form the 

 basis of this rule by means of the Abbe diffraction plate. If any line- 

 system of the plate be so focused that the central image of the whole 

 diffraction spectrum visible in the focal plane of the objective lies in tho 

 axis of the Microscope (direct illumination), and one entire half of the dif- 

 fraction spectra be then screened off by a suitable diaphragm (with the 

 exception of the central image for which the semicircular diaphragm 

 must have a piece cut away), the microscopic image will not suffer any 

 essential change. It is also possible, as may be easily seen, to set the 

 mirror so obliquely (or to obtain oblique illumination by Abbe's con- 

 denser), that the rays which have not been diffracted still contribute to 

 the microscopic image, the image of the source of light then falling at 

 the margin of the diffraction phenomena visible through the tube. In 

 this case still further diffracted rays may become visible in the diffraction 

 image, and may contribute to the delineation if such rays are present to 

 a considerable extent. 



I cannot help calling attention to two other possible sources of 

 error. It is not impossible that the discs of unequal index of which the 

 muscle-fibre is constructed, are not separated from one another by sharp 

 boundaries, but the optical density may change gradually from one to 

 another. Such layers have in fact been described. 



Now a disc in which the refractive index is a maximum or a minimum 

 at the centre, acts like a cylindrical lens upon light which enters it 

 parallel to its plane ends (independently of the cylindrical surface). 

 The parts of a wave surface which traverse layers of smaller index, 

 travel more rapidly than those which have to traverse layers of greater 

 index, so that there results a cylindrical curvature of the wave surface.* 

 In this way focal lines may be produced which are parallel to the layers 

 in the muscle ; they need not be outside the fibres, but may lie within 

 them ; in the first case they alter their position as the thickness of the 

 fibre increases. 



It is evident that stripes which are produced in this way, as well 

 as those which result from diffraction, must undergo various changes 

 if an alteration takes place in the refractive indices, owing to the separa- 

 tion of a liquid from the muscle-fibre. Since such changes do take place 

 during the life of the muscle, it is not a matter for surprise if the fibres 

 which are still contracting change their appearance. Rollett has in 

 fact described and figured a series of such changes, but whether they 

 are due to the causes here indicated, I must, in the presence of such a 

 number of possibilities, leave undecided. 



Mention has repeatedly been made of darker and lighter layers 

 in the fibre, and Eollett, in treating of the transverse striations of the 

 fibres, likes to give two figures beside one another, one taken with high, 



» Cf. S. Exner, ' Ueb. Cylinder welche optische Bilder entwerfen.' This Journal, 

 1886, p. 1062. 



