The Structure of Diatoms. By Dr. J. H. L. Flogel. 683 



proximity an observable diffraction phenomenon, are not imaged 

 geometrically in the Microscope, that is to say, not as if the homo- 

 focal emergent pencils of rays from the object represented it pomt 

 for point on one image-surface." From this he draws the con- 

 clusion (p. 453)— "all attempts to determine the sculpture of the 

 finer diatom-valves by morphological interpretation of their micro- 

 scopical images seem founded on inadmissible premises. Whether 

 Pleurosigma angulatum has two or three systems of strias or 

 whether real strife are there at all, or whether the observed mark- 

 ings are caused by isolated elevations or isolated depressions no 

 Mfcroscope can determine however perfect it may be or however 

 strong its magnifying power." Further (p. 454)—" that the same 

 condition of things exists very nearly for a great number of purely 

 organic images in histological work, can be learnt by the example 

 of striped muscular fibre. In good preparations the diffraction 

 phenomena can be easily observed, and their effects in the micro- 

 scopical image can be studied experimentally in the former-described 

 manner. The manifold differences in the character of the image 

 explain to some extent the disputes which have arisen between 

 different investigators on this point ; but at the same time they 

 also establish the impossibility of stating anything defimte about 

 their real organic composition in the sense of the attempts made 

 hitherto." ,. ^, 



I am not aware whether Professor Abbe still clmgs to these 

 views expressed in 1873, or whether he bas since convinced himself 

 of their error. From his pubHcations which 1 have since occa- 

 sionally seen, I believe he still holds to the former opinion. These 

 theses figure as principal results in a journal of eminence, which 

 must be read by everybody who wishes to keep an account of what 

 he sees in his Microscope. Therefore I consider a refutation of 

 these theses in this place a necessity. 



Smce the structure of muscular fibre and the differences 

 amongst histologists of that date are put forward as examples of 

 the correctness of the assertions, it may be well to bear in mind 

 that the greater number of histologists have not adopted in their 

 researches Prof. Abbe's views; and that now-a-days the com- 

 plicated structure of the transversely striated muscle-fibre is nearly 

 estabUshed. This is not only valid as regards the single layers 

 composing the fibre, but also for the double-refraction of certain 

 parts, of which Abbe also states (p. 453), it was futile to entertain 

 the idea. I will not here enter into the full details how, at the 

 commencement of the last decade, the confusion chiefly brought 

 aljout by lleppner's wrong views about the muscle structure was 

 dispelled by my work (8), based on the examination of an unusually 

 favourable object, and 1 hkewise demonstrated at the same time 

 that with the application of good hardening methods one can 



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