132 SUMMARY OF CURRENT RESEARCHES RELATING TO 



more recently investigated experimentally by Quincke and theoretically 

 by Jocbmann.* 



The phenomena are, beyond comparison, more complicated in the 

 muscle-fibres, as must be at once apparent if it is remembered that 

 the conditions described do not depend upon a b being the surface of the 

 fibre, so that the above treatment holds good for any plane within the 

 fibre for which the portions of the wave that traverse the different discs 



have a difference of phase equal to - ; and when it is remembered also 



A 



that the phenomena must change with the thickness of the layer, that 



the source of light is not a point, but a bright surface (a portion of the 



sky or its image), that the light used is mixed light, &c. 



The case may also be made clear in the following way : — When a 

 plane wave traverses discs of unequal refractive index, it acquires 

 parallel ridges corresponding to the layers of smaller index. The 

 problem then consists in the determination of the resultant of the inter- 

 ference of the elementary waves proceeding from a surface of this form. 



Some years ago Heppner \ suspected that a certain layer of the 

 muscle-fibre, identical with Rollett's N, does not in reality exist, but is 

 confused through a reflex. Sachs % and others opposed this idea. 

 Donitz § seems to have been the first who thought of diffraction 

 phenomena as the explanation of certain striations. He was followed 

 by Schafer, and Eanvier made experiments upon the diffraction spectra 

 obtained from stationary and contracted fibres in which the transverse 

 striations acted as a diffraction grating. 



I have, in the above remarks, raised the question whether, in the 

 light of this optical treatment, the results of recent investigations have 

 any value as regards the distinguishing of several layers in the muscle- 

 fibres where previously two alone were recognized, or whether we must, 

 with Abbe, for ever despair of recognizing such minute details. 



My answer amounts to this, that without doubt the greater part of 

 the recent results deserve complete trust. All those layers which have 

 been distinguished, not only in the optical image, but also by maceration 

 and staining experiments, are free from the suspicion of being only the 

 impression of incomplete delineation. Eollett, who seems to have been 

 thoroughly aware how slippery is the ground of simple microscopic 

 examination, has recently, as I think, trodden the path here indicated 

 with the best results. The same has been attempted, it is true, by 

 many inquirers before him, but no one has worked in this direction with 

 such a variety of methods or obtained such promising results. 



When for example the layer N under the action of acid behaves in 

 an essentially different way from the layer Q, there can be no doubt 

 that a distinction is here established. But the case is different with 

 certain details, where one meets with the above-mentioned want of care 

 against incomplete delineation, in consequence of which one can see even 

 more than is really present. I may be here allowed to give examples ; 

 but I may first state that in the absence of a true criterion for a correct 

 and complete representation of the object, the following may serve as a 

 criterion. A detail of the microscopic image is to be regarded as 



* Cf. Verdet, ' Vorlesungen iiber die Wellentheorie des Lichtes.' German transla- 

 tion, by K. Exner, i. (1881). | Arch. f. Mikr. Anat., v. (1869). 

 X Du Bois Reymond and Reicherfs Arch., 1872. § Ibid., 1871. 



