ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 305 



the perception of a colourless object is more distinct in proportion as 

 the difference is greater between the refractive indices of object and 

 embedding medium. His experiments were mainly with the cortical 

 fibres of certain nettles {Boelmeria tenacissima Gaud.), and the media 

 were a cinnamon oil of refractive index 1'597, and benzyl-alcohol sp.gr. 

 1 • 540. The following is a resume of his conclusions. 



1. An image of a colourless object can only be formed under the 

 Microscope when there exists a difference of refractive index of object 

 and surrounding medium. If object and medium are optically isotropic 

 only a single value of this difference has to be considered. If the object 

 is doubly refractive and if the actual section through the index ellipsoid 

 in the object plane is an ellipse, then there must be two limiting values 

 corresponding to the semi-axes of the ellipse. If these two different 

 values vary from zero, diffraction spectra must exist winch usually do not 

 resemble each other. The interference efforts in the image plane must 

 correspondingly vary and the microscopical image is due to their super- 

 position. 



2. If, however, one of these difference values is zero, no diffraction 

 spectra can exist, and therefore no image of the object. If the refractive 

 index of the isotropic embedding medium be expressed by n and the 

 two refractive of indices the object by % and n 2 , then either n l - n or 

 n. 2 — n can equal zero. In the first case the image depends on the 

 difference « 2 — n and in the second case on n 2 — n . The rays corre- 

 sponding to the values n v n 2 , as a result of the laws of double refraction, 

 are polarized perpendicularly to one another. 



3. If a bast-fibre of the nettle plant be chosen as object and w 3 

 corresponds to the semi-axis of the index ellipse lying parallel to the 

 bundle-axis, and n 2 to the one perpendicular to it, then ?? T 7 ?i 2 . If 

 such a fibre be observed in benzyl-alcohol (ref. ind. = 1*540) over a 

 polarizer, then usually no image is formed if bundle-axis and polarizing 

 plane of the Nicol are parallel. If both directions are crossed, then a 

 distinct image of the fibre is found by means of a ray which is polarized 

 perpendicularly to the bundle-axis. If the observation be made in 

 cinnamon oil (ref. ind. 1"597) the contours disappear if bundle axis and 

 polarizing-plane are crossed, and the image is formed, when both 

 directions are parallel, by means of a ray polarized parallel to the 

 bundle-axis. 



4. From what has been said in (3) it follows that the fibre acts as au 

 analyser in both cases. If between the fibre and the polarizer a selenite 

 plate be diagonally inserted, the contours will appear in two positions, 

 inclined at 90° to one another in the same colours. 



5. Contours which in a bright field appear dark on a bright ground 

 must in a dark field appear bright on a dark ground. The ray of the 

 diffraction spectrum which through interference in the bright field pro- 

 duces a minimum of intensity must in the same position in the dark 

 field attain a maximum of intensity. On the insertion of a selenite plate 

 the colours in the two fields are complementary. Hence, by super- 

 position of the images cast by the two fields combined with a little 

 weakening of the central beam, the microscopical image can be usually 

 made to disappear. 



