38 MUSCULAR FIBRES IN POLARISED LIGHT, BY E. BRUCKE. 



in the direction of its length in a groove h h ; the lower one 

 fixed, and only movable together with the slide by means of the 

 micrometer screw. The prisms rest upon their thin edges, and the 

 stage is perforated immediately beneath them, so that the light is 

 freely transmitted. They have both a corresponding angle of 

 1 6' 54", and are so cut that one of the inclined planes in each 

 is parallel to the crystallographic principal axis, so placed that 

 the light reflected from the mirror of the microscope passes 

 perpendicularly to this axis in each, and so arranged that the 

 two principal axes cross each other at right angles, each of them 

 making an angle of 45 with the polarising plane of the sub- 

 jacent Nicol's prism. Since the two prisms act in a contrary 

 sense, so that the ray which is ordinary in the first becomes extra- 

 ordinary in the second, there are obtained, if the Nicol's prism 

 situated above the ocular be made to cross that which lies below 

 the quartz, a few black strise, where equal thicknesses of the 

 latter lie over one another, whilst on both sides colours appear in 

 the sequence of the Newtonian system of rings for reflected light. 

 The prisms, moreover, by sliding can be so arranged that the 

 black stria which is present when the differences of velocity of 

 the rays = 0, or the colour corresponding to some determinate 

 difference of the ray, can be made to occupy the middle of the field. 

 I now make the upper of the two quartz crystals an object 

 stage, and distribute the muscular fibres of Hydrophilus piceus 

 upon it in such a mode that, whilst some lie parallel to the 

 principal axis, others are arranged perpendicularly to it. If 

 the micrometer screw is now turned so that an increasingly 

 thick portion of the lower prism is gradually brought into the 

 field, it will be remarked that each colour is first assumed by 

 those muscular fibres which are arranged at right angles to the 

 axis of the upper prism, then by the ground, and lastly by the 

 muscular fibres which lie parallel to the axis of the upper prism. 

 If the screw be turned in the opposite direction, these colours 

 are first shown by those muscular fibres which lie parallel to 

 the axis of the upper prism, then by the ground, and then 

 by the fibres which stand perpendicularly to the axis of the 

 upper prism. Every muscular fibre . therefore acts optically 

 like a thickening of the prism to the axis of which it is 

 parallel, or, which is the same thing, as an attenuation of the 



