584 



SUMMAKY OF CURRENT RESEARCHES RELATING TO 



of rays transmitted through the film of fluid at C ; and by properly 

 regulating this amount, the relative dispersion of the fluid and the 

 flint-glass of the prisms may be exactly compensated for, or balanced, 

 and the dividing line of the ocular field made to appear colourless 

 even with white light. At the same time the position of the com- 

 pensating Amici prisms (the angle z with which the compensation is 

 obtained) aiibrds all necessary elements for computing the dispersion 

 of the fluid under investigation, provided the refraction and the 

 dispersion of the glass-prism A is exactly known from previous 

 measurement. The author shows that the dispersion of the fluid 

 (for any definite interval of the spectrum) may be obtained by means 

 of the formula 



8 w = A 4- B 0-, 



where A and B are co-efficients which depend on the refractive index 

 of the prism A and of the fluid, and the dispersive power of the 

 prism A at the same time, whilst a depends on the angle z of the 

 compensating prisms only. The co- efficients A and B may be easily 

 computed for every instrument and arranged in a tabular form, a 



definite interval of the spec- 

 FiG. ll.'). ,ss^ trum (e. g. from D to F) being 



assumed for 8 n. 



Fig. 115 shows the instru- 

 ment complete. FJ is the 

 small telescope magnifying 

 two or three times. In the 

 focus F of the object-glass 

 are the double cross threads. 

 At the upper end the telescope 

 is attached to a tube, in which 

 the eye-piece (consisting of 

 a convex lens) slides. At its 

 lower end the telescope is 

 screwed to T D S, containing 

 the two j)risms of the compen- 

 sator ; D again is fixed to the 

 sector A. The two prisms 

 between which the fluid is 

 placed are at C. One of these 

 is fixed to an axis which passes 

 through the support G, and 

 has also attached to it at K an 

 alhidadc or movable index-arm 

 B. The refracting edge of the 

 prism is at right angles to the 

 plane of the sector and the axis 

 of the telescope. The second prism is simply ledged on the former, 

 being held in place by a spring. The graduation on the sector is 

 arranged so as to give the refractive index directly, and shows 

 thousandths, so that with the naked eye the refractive index to three 



