202 H. von Mohl on the Investigation of Vegetable Tissue 



in this case the field appears hght, as in the ordinary micro- 

 scope. 



If, when the Nicols are in a crossed position, we place a simply- 

 refracting body, for instance a crystal of common salt, in the 

 focus of the microscope, this exerts no influence on the polarized 

 light coming from the lower Nicol : the light passes through the 

 body freely, without suffering any alteration of the direction of 

 its vibration, and is incapable of arriving at the eye through the 

 upper Nicol ; so that the body is not seen. But the conditions 

 are essentially different when a doubly-refracting body, e. g. a 

 plate of mica, selenite, &c., is placed in the focus of the micro- 

 scope. When such a body is revolved in a horizontal direction 

 round the axis of the microscope, it will behave in four positions 

 like a simply-refracting body, and be invisible ; this invisibility 

 will supervene anew each time the body is rotated a quarter of 

 a circle from one of these positions, while in the intermediate 

 positions the body will be visible. If now in each of these four 

 positions in which the body is invisible, we draw in imagination, 

 or in reality, upon it, passing through its centre, a line which 

 stands perpendicular to one of the two Nicols, we find upon the 

 body two lines crossing at right angles, which are designated as 

 the neutral axes. In using this name, it must be kept in view, 

 that it implies not only the lines drawn through the centre of 

 the body, but also those which run parallel with them, and that 

 these denote the two directions, crossing at right angles, of which 

 one is parallel to the direction of vibration of the polarized light, 

 the other perpendicular to the same, when the object is invisible. 

 Therefore when it is desired to draw a figure of these axes, they 

 must not be represented in the form of two lines crossing at 

 right angles, but as two systems of parallel lines crossing each 

 other at right angles. 



If the doubly-refracting body is placed in any other position, 

 it will be visible through the upper Nicol, and with a light pro- 

 portionately brighter, as the angle under which its neutral axes 

 are inclined toward the transverse diameter of the Nicol, ap- 

 proaches an angle of 45°. The undulatory theory explains 

 this becoming-visible of the body by the assumption that the 

 polarized light, when its plane of vibration is not parallel with 

 one of the neutral axes of a doubly-refracting body, is divided 

 in its passage through the latter into two portions, whose planes 

 of vibration are perpendicular to each other, whereby the light 

 modified in this way acquires the power of passing, certainly with 

 more or less diminished intensity, through the upper Nicol, and of 

 rendering the body in whose interior this division of the light into 

 two bundles occurs, visible, as an apparently self-luminous object*. 

 * The reader who desires more detailed information respecting the 



