In the center of the apparatus a metal cylinder, _A_, is found carrying two plate glass pieces, _C B_, so placed as to form an angle of 107°. This cylinder is placed in a larger one, provided with two circular glass windows. To these two openings are fixed to the right and left, the telescopic attachments, _G_, _V, S, E_, and the apparatus _M, H, Sʹ_, _Eʹ_, for rendering the rays of light parallel. The field of vision is divided into two portions, light and dark, by a semicircular stop inserted in the collimator, and contains the double scale shown in the figure placed at _H_. The field of vision is illuminated by a gas or oil lamp placed at a convenient distance from the collimator. The inner metallic cylinder _A_ is surrounded with an outer one, to which the optical parts are attached at _D Dʹ_ by means of plane glass plates. This cylinder is in turn contained in the large water cylinder _P P_, carrying a thermometer in the opening shown at the top on the left. The manipulation of the apparatus is very simple. The outer cylinder is filled with water, at a temperature below 22°, the middle one with the typical oil furnished with the instrument, the cover of the apparatus carrying the thermometer placed in position and the cup-shaped funnel inserted in the cylinder _A_, which is at first also filled with the typical oil. The whole system is next brought slowly to the temperature of 22° by means of the lamp shown in Fig. 92. The telescope is adjusted to bring the scale of the field of vision into focus and the line dividing the light and shadow of the field should fall exactly on 0°_a_. If this be not the case the 0° is adjusted by screws provided for that purpose until it is in proper position. The typical oil is withdrawn from _A_ by the cock _R_, the cylinder washed with a little of the oil to be examined and then filled therewith. On again observing the field of vision the line separating the shadow from the light will be found moved to the right or left, if the oil have an index different from that of the typical oil. The position of the dividing line is read on the scale. For fats the temperature of the apparatus is brought exactly to 45° and the scale 0°b is used. In other respects the manipulation for the fats is exactly that described for oils. In the use of 0°a, in case the room be warmer than 22°, all the liquids employed should be cooled below 22° before being placed in the apparatus. It is then only necessary to wait until the room temperature warms the system to 22°. In the case of fats it is advisable to heat all the liquids to about 50° and allow them to cool to 45° instead of heating them to that temperature by means of the lamp.