ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 583 



Witli white light the boundary line between the dark and bright 

 parts of the field of view is coloured on account of the difference 

 between the refractive indices of the differently coloured rays, a fact 

 which is made use of to determine the dispersive power of the fluid. 



For this purpose a " compensator " is added consisting of two 

 direct vision (or Amici) prisms, E and S fig. 114, so constructed 

 that rays of a given colour D will pass through each without 

 deflection, whilst rays of a different colour will be deflected towards 

 the former, and make with it an angle h in the direction of the 

 principal section. By turning the screw head T of the pinion acting 

 on a circular rack at P the prisms simultaneously revolve in opposite 

 directions, and starting from the position shown in the figure (the 

 " primitive plane ") in which the two principal sections are parallel, 

 and the refracting edges directed to the same side, they pass through 

 equal angles in opposite directions, so that 

 the two principal sections remain always Fig. 114. 



symmetrically inclined to the primitive plane. t 



The principal sections will again coincide nTr" 



after revolving 90°, 180°, &c., with this differ- j V 



ence, however, that at 90° the refracting ( — ■ ' ' > 



edges will lie in reversed directions, whilst 



at 180° both will be in the same direction, i 



the opposite of that which they occupied in 



the original position. ' ' ' 



It follows, therefore, that the rays cor- 

 responding to the line D will pass through the prisms (provided 

 they can pass at all, i. e. are not totally reflected) without deflection, 

 whatever their relative position, and that all other colours will undergo 

 deflection only in that plane in which the two prisms coincide, i. e. 

 the principal plane. During the revolution the extent and direction 

 of the dispersion within that plane varies for any two colours as the 

 diagonal of a parallelogram whose sides are proportional to the dis- 

 persion k of the single prisms, and correspond with the direction, 

 whatever it may be, of the principal sections. 



Hence the " compensator " acts as a single direct-vision prism 

 for the colour D, with a constant principal section, but variable 

 dispersion within that section ; and in every position of the prisms, 

 by their revolution (either way) through an angle z, the amount of 

 their dispersion for the assumed colour, and for all others proportion- 

 ately, is 



K = 2 k cos z. 



Consequently this amount may have all values from 2 7c to -|- 2 A;. 



If we look through such a combination of prisms at a luminous 

 line perpendicular to the central plane of the principal section, it 

 will be seen to extend into a spectrum constantly lengthening as the 

 prisms revolve, becoming contracted again, as the revolution is con- 

 tinued, to a colourless image, and again expanding into a spectrum 

 with the colours in reversed order. 



By means of this device any desired amount of dispersion 

 between the limits — 2 7c and -\- 2Jc may be introduced into the pencils 



