ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 



587 



Fig. 116. 



tains a description of a new form of " spectrometer," for the exact 

 measurement of the refractive indices and dispersive powers of 

 prisms, which, however, is mainly of interest to physicists. 



Professor Abbe subsequently suggested * an addition to the 

 refractometer, in order to enable the refractive indices of solid bodies 

 to be determined. For this purpose a small part of the metal back 

 of the fixed prism is removed and a small prism is cemented on. 

 A piece of the substance to be examined is then attached to one of 

 the faces of the large prism by a fluid of high refractive index. 

 In fig. 116, A is the prism of the refractometer, S the solid substance 

 — a plate with one surface polished and cemented to the face C of 

 the large prism by means of a drop of cassia-oil, or monobromide 

 of naphthaline ; a, the small prism cemented to A, in order to admit 

 rays from above to the face 

 C. If the angle y of the 

 incident rays at C, on 

 rotating the prism A, be- 

 comes equal to the critical 

 angle for the substance S 

 — relatively to the flint- 

 glass of A — total reflection 

 begins, and the under half 

 of the ocular-field appears 

 hrightly illuminated, the upper half less so. The line of intersection 

 being brought just upon the cross-wires and made colourless by 

 means of the compensator, we have again the same conditions as in 

 the case of transmitted light. 



In order that the total reflection shall be obtained from the solid 

 substance S, and not from the fluid film by which it is cemented to 

 A, the refractive index of this fluid must of course exceed that of 

 the solid under investigation. 



Refractive Indices and Dispersive Powers of Solids and Fluids, t 

 — Professor L. Matthiessen of Eostock gives a table of refractive 

 indices and dispersive powers, prefaced by the following observa- 

 tions : — 



" It is well known that recently in practical optics the theory and 

 technic of microscope objectives have attained special interest. Im- 

 portant advances in the perfecting of the Microscope are connected 

 with the names of Helmholtz, Abbe, Amici, and Stephenson. The 

 problem of eliminating spherical and chromatic aberration in objec- 

 tives has through them (in different directions) been brought nearer 

 a solution, although in the optical properties of transparent solid and 

 fluid bodies there are still many obstacles to further perfection to be 

 overcome. In the substances hitherto observed the dispersive power 

 generally increases with the refractive index, and substances are wanted 

 which combine small dispersion with a high refractive power or vice 

 versa. Moreover the fluids hitherto observed all have a propor- 



* " Ueber die Bestimmung der Brechungs-Verhaltnisse fester Korper mittelst 

 des Kefractometers," SB. Jeiiaisch. Gesell. f. Med. u. Naturwiss., 1879, pp. 35-44. 

 t Centr. Ztg. f. Opt. u. Mech., iii. (1882) pp. 73-4. 



