ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 129 



therefore be surrouuded by a dark ring ; all points wbicb satisfy the 

 same conditions as m and whicb lie in the plane of the figure belong to 

 a hyperbola whose apex lies in a b ; as the tube is raised the dark ring 

 will therefore increase. 



According to the conditions e will be surrounded by a certain number 

 of dark rings corresponding to differences of path, which are an unequal 

 number of half wave-lengths, and between them will lie bright rings. 

 These diffraction phenomena may be well seen with particles of Indian 

 ink in water when a round opening of 1 cm. in a screen before a gas- 

 flame is used as illuminator; the same thing may also be seen with 

 ordinary illumination. 



Certain interference-bands lie in the immediate neighbourhood of 

 the object, and are seen when the Microscope is focused close to the 

 object ; and when the latter has, as is the case with the muscle-fibres, a 

 considerable thickness, the diffraction images may even lie inside the 

 object, aud thereby considerably increase the danger of error. Now, as 

 'has been said above, the image is no longer reliable when the object 

 attains a certain minuteness, so that in such cases it may be uncertain 

 whether the Microscope is focused on the object or on the diffraction 

 appearances. As is well known, the different interpretations put by 

 Engelmann and Meyer upon the process of contraction in muscle-fibres 

 depend on the different modes of judging what is meant by the ' true ' 

 focal adjustment of the object.* 



In working with the Microscope we see every day examples of these 

 diffraction images ; a sufficiently minute drop of mastic emulsion has 

 naturally a definite outline and a transparent interior, like a larger drop, 

 but this cannot be seen ; in general, what is seen is a dark point, or with 

 a different focus a bright point surrounded by a dark circle. Whether 

 the object consists of a transparent liquid or a black pigment we cannot 

 say, since the diffraction phenomena are the same in the two cases. With 

 a sufficiently fine thread a similar figure is produced. 



The practised microscopist, although he only sees the diffraction 

 phenomena, and even in consequence of them, will realize the existence 



* Cf. Merkel in Arch. f. Mikr. Anat., ix. (1873) p. 299. Merkel here attempts 

 to settle the question by examining the primitive fibrilke in polarized light, and 

 since the ordinary illumination gives no result he employs direct sunlight. I cannot 

 regard this as satisfactory, for in this case the small angular size of the source of 

 light introduces conditions peculiarly suitable for diffraction phenomena. In fact it 

 is impossible to ignore the fact that if the double-refraction has not been essentially 

 altered in the balsam preparations, and there is no reason to believe this to be the 

 case, Merkel's results cannot be attributed to this cause; a single fibrilla is too thin. 



If the fibrilla is only visible in blue light upon a dark field the difference of path 



of the two rays must amount to — of this light. According to Ketteler, for the line 



G in vacuum 



\v = 0-000430409 mm. 



So that with the above values of n for the ordinary and extraordinary rays in a 

 living muscle-fibre 



\o - 0-00031463 mm. 



Ae = 0-00031417 mm. 



Assuming for the fibrilla the considerable thickness 0-002 mm. it contains 6-356 

 waves of the ordinary and 6 ' 366 waves of the extraordinary ray ; that is, the differ- 

 ence of path is only 1/100 of a wave-length ; and this is not in harmony with the 

 effect described. 



1888. k 



