434 MANIPULATION AND PRESERVATION OF THE MICROSCOPE 



such in small objects. The microscopical image might easily lead 

 us to the conclusion that we were examining a cylindrical body 

 composed of bells or funnels inserted one in another. The spirally 

 thickened threads, for instance, as they originate from the epidermis 

 cells of many seeds, were thus interpreted, although here and there 

 by the side of the irregular spirals quite regular ones are also 

 observed. In illustration -of this a very excellent example is given 

 in the'Quekett Journal' for 1899 (No. 44), p. 166, where Mr. 

 Nelson shows that a certain structure in the 

 remarkable diatom Climacosphenia moniligera, 

 which for a long time has been regarded as inter- 

 locking teeth, is in reality a spiral pipe. 



Moreover, it must not be forgotten that in 

 the microscopical image a spiral line always ap- 

 pears wound in the same manner as when seen 

 with the naked eye, while in a mirror (the inver- 

 sion being only a half one) a right-handed screw 

 is obviously represented as left-handed, and con- 

 versely. If, therefore, the microscopical image 

 is observed in a mirror, as in drawing with the 

 Sommering mirror, or if the image-forming pen- 

 cils are anywhere turned aside by a single reflec- 

 tion, a similar inversion takes place from right- 

 handed to left-handed, and this inversion is again 

 cancelled by a second reflexion in some micro- 

 scopes. All this is, of course, well known, arid to 

 the practised observer self-evident; nevertheless many microscopists 

 have shown that they are still entirely in the dark about matters of 

 this kind. 



One of Professor Abbe's experiments on diffraction phenomena 

 proves that when the diffraction spectra of the first order are stopped 

 out, while those of the second are admitted, the appearance of the 

 structure will be double the fineness of the actual structure which is 

 causing the interference. 1 



FIG. 367. A spiral 

 in motion. 



FIG. 368. 



Upon this law there appears to depend a number of possible 

 fallacies, errors which may arise from either its misapprehension or 

 misinterpretation. At least these appear to us, from a practical 

 point of view, to be of sufficient importance to need either caution 

 or a fuller exposition of the great law of Abbe in regard to them. 



If, for example, figs. 368, 369, and 370 may be taken to represent 

 1 See Chapter II. 



