ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 125 



all the other spectra are shut off with the exception of two belonging to 

 the series a t a 3 , a. 2 a 4 . . . a± a 6 . The striation IX. is then repeated twice 

 by a 2 a 6 and a 3 a 5 . 



Since the distance of the spectra a a 1} a a 2 ... or a i a 3 , a 2 a 4 . . . is 

 greater in the ratio 1 : \ 3 than that of a a u a a 2 . . . the lines of the 

 stria? VII.-IX. must be closer to one another than those of I.— III. in the 

 ratio v 3 : 1. The new striations IV.-IX. called into existence by the 

 above arrangements possess the same sharpness of outline as those which 

 have been long known, namely I.-III. 



The above appearances serve to explain the different views which 

 have been held with regard to the structure of diatoms, when they are 

 observed with different modes of illumination. Dry and water-immersion 

 objectives of no great numerical aperture show the well-known hexagons 

 (plate III. fig. 3) when the illumination is central and with a not very 

 minute diaphragm, or when the illumination is oblique if e. g. a a, a 2 a 3 

 rOV a rtj a 5 a s are operative. Large numerical aperture with central 

 illumination gives bright circles arranged in lines which intersect at 60°, 

 and between which with very sharply defining objectives (homogeneous- 

 immersion for instance) dark spots are also visible (plate III. fig. 4). 

 Oblique illumination and the action of a, a 2 a 3 , a x a 5 a , with a numerical 

 aperture up to 1 • 10 shows a chess-board pattern as described by Schiff 

 and Dippel (plate III. fig. 5). Very oblique illumination and the action 

 of a a 2 a 3 a 2 or a a 5 a 6 a 5 with objectives of very large numerical aperture 

 give the peculiar figure first observed by Stephenson and Aobe, in 

 which the bright rectangular spaces are traversed by a small dark line 

 and are accompanied by dark markings equal to the first in size and 

 lying above and below them (plate III. fig. 6). Other forms may be 

 obtained on a bright or dark field by the use of various modes of 

 illumination and of diaphragms which intercept certain spectra of the 

 first and second series and only allow the remainder to operate. 



That the ordinary markings which are seen with an objective of large 

 numerical aperture and with central illumination are more nearly related 

 to the true structure than the other images, can only be concluded from 

 conditions of their production, and not from the images themselves. 

 These markings appear when the largest possible part of the total 

 spectrum of the Pleurosigma valve is in operation, and as little as 

 possible (i. e. only the furthest fainter pencils of the second and third 

 series) is lost ; while each of the other images is produced by a much 

 smaller part of the total diffraction spectrum. For this reason it may be 

 concluded that the former image is less dissimilar than the others from 

 the image which corresponds to the complete diffraction action of the 

 valve, and which is unattainable by any Microscope.* 



(3) Prof. Exner's remarks on the Optical character of living Nnscle- 

 fibres.\ — Prof. S. Exner employed his micro-refractometer J to determine 

 the refraction and double refraction of living muscle-fibres, and to 

 answer the question whether transversally-striated fibres have their 

 refractive index increased or diminished during contraction. The paper, 

 as we have above stated, is more particularly interesting to microscopists 

 from the observations which the author makes on the application of the 



* Dippel's Das Mikroskop, 1882, pp. 158-61 (6 figs.). 



t Arch. f. d. gesammt. Physiol. (Prliiger), xl. (1887) pp. 360-93 (2 pis.). 



% See this Journal, 1886, p. 328. 



