Exhibit 265 is the Abbe refractometer which uses total internal reflection to determine the refractive index of a sample. In use a small sample of the substance for which the refractive index is to be measured (a drop of liquid or a small fragment of a solid) is placed on the hemisphere of flint glass which appears on the top of the instrument. Toward this point is oriented an adjustable source of monochromatic light from one side and a telescope from the other side.
As indicated in the schematic diagram, for incident angles i less than the maximum angle l, the reflected rays are weak. For incident angles greater than l, the reflection is total and thus the reflected rays are more intense. In the field of view of the telescope focused at infinity, there is a neat separation corresponding to angle l, between the bright and dark parts. The angle l is measured on the graduated scale to a precision of one minute with the help of a vernier. Then Rg*sin i=Rs where Rg is the refractive index of flint glass, Rs is the refractive index of the unknown substance, and i is the critical angle or that of total internal reflection.
The apparatus consists of:
a hemisphere of flint glass of refractive index between 1.75 and 1.9
a protractor for measuring angle l with a vernier.
The choice of flint glass is due to its high index of refraction since this method works only for substances with lower indices of refraction. The liquid connot be an acid which attacts flint glass.
ISCRIZIONI
Sullo strumento sono incisi il logo del costruttore e (subito sotto) il numero 5708
A lato è inciso il numero 1,749, probabilmente l'indice di rifrazione del vetro di cui è fatta la semisfera
BIBLIOGRAPHY
[1] E. Perucca," Fisica Generale E Sperimentale" Vol.II, UTET, Torino, 1937. Pagina 337.
