BY DR. KLEIN. 69 



We have now to consider the significance of the appearances 

 above described. The fact may be stated in limine, that the 

 whole of what intervenes between two interstitial stripes, /. ,-., 

 the gray band and its two bright borders, affects polarized 

 light in the same way that consequently the view according 

 to which only the borders are doubly refracting, is erroneous. 

 If a microscope is employed, of which the stage admits of ro- 

 tation around the vertical axis of the instrument, as in the 

 larger instruments of Hartnack, and these bright borders 

 (which should be distinct and regular) are observed, in a 

 muscular fibre, under No. 8 objective, and if the stage is slowly 

 rotated, so as to alter the course of the rays in relation to the 

 muscular fibre under observation, a remarkable change is seen 

 to take place in these borders. As the rotation is continued 

 the bright bands fade, first on one side of the interstitial line, 

 then on the other, coming into view again in the same order; 

 the changes dtf not, however, occur simultaneously through- 

 out the whole of the muscular fibre. 



When fresh muscular tissue is placed in absolute alcohol, 

 and then steeped for a few minutes in oil of turpentine, 

 mounted in dammar varnish, and covered, only two kinds of 

 substance can be distinguished in the fibres, i.e., dark inter- 

 stitial stripes and a dull gray substance between them, without 

 a trace of the clear borders. The longitudinal section of such 

 a muscular fibre ma}' be represented diagram maticall}', as in 

 Fig. 15. In the diagram, the slightly refracting interstitial 

 substance is represented by a, the clear borders by 6, the dull 

 gray by c. Let us endeavor to understand the course of the 

 ra3 T s which pass through b. Let j be a ray which enters from 

 the mirror in the direction a j3, and penetrates at o into the 

 less refractive medium a, and passes through it in the direc- 

 tion o 5 inasmuch as it deviates from the normal in a. Let 

 $' be another ray which enters b at the angle m' ', i. e., a greater 

 angle than that under which $ enters. Accordingly, the de- 

 viation it will undergo in the medium a will be greater than 

 the deviation undergone by $. And if it be assumed that it 

 is so great that the sine of the angle of dcviation=f, the angle 

 being a right angle, the ray will pass out between a and 6. If 

 the angle of incidence it greater than in', the angle of deviation 

 is greater than a right angle, so that the ray does not enter a 

 at all, but is totally reflected through b. Hence the substance 

 b appears clearer than c, for more rays pass out at b than at c; 

 the excess consisting of those rays which, entering b in a di- 

 rection towards a, and at a greater angle than ??i', are totally 

 reflected in 6. as above explained. 



The bright borders of the proper substance present the same 

 characters and relations in the muscles of crabs after treat- 

 ment with gold. Occasionally they may be also seen in the 



