AT HIGH PRESSURES BY OPTICAL METHODS. 
125 
the iron plate to which the water-jacket and pressure-ljomb are attached. With tlie 
aid of an antocolliination Gauss’ eye-piece, the whole optical bench can, by means of 
the screw-adjusting gear, be brought exactly into such a position that its optical axis 
is at right angles to the surface of the window in the pressure-bomb, oi' any polished 
surface inside of tliis. The adjusting gear for the optical bench works practically in 
the same way as the crystal adjustment on a modern goniometer for measuiing 
crystals, except that it, of course, can be adjusted at only a very small angle, and 
instead of the crystal or refractive index prism being adjusted on a goniometer, the 
object to be measured here remains in a fixed position, and tiie whole optical 
apparatus is adjusted with regard to this fixed position. 
The optical part itself consists of an illuminating lens, polarizing Nicol-prlsm, 
condenser, objective, analysing Nicol-prism, and micrometer eye-piece. These optical 
parts are very much the same as those of the well-known Fuess Goniometer No. II. 
For crystal optical measurements the eye-piece can be replaced l)y a compensator eye¬ 
piece, and a stronger condenser-lens and objective-lens can be clam]3ed on to the ends 
of the eye-piece and collimator-tubes, just as in the case of tlie Fuess Goniometer II., 
when used for optical axial-angle measurements. For such a purpose the eye-piece 
tube is also provided with a “ Bertrand lens.” The eye-piece system is focussed on 
infinite distance, but can be converted into a microscope of small enlarging power by 
means of an attachable lens. Thus, with this optical bench, tlie same measurements 
can be carried out as with the optical part of the Fuess Goniometer II. The 
description of the use of it for the determination of the refractive index and the 
dispersion of compressed liquids must be left for a paper dealing with this question. 
The optical bench and other optical parts were made liy B. Fuess, of Steglitz, Berlin. 
The water-jacket with its fittings, and the gear for adjusting the optical bench, were 
made by the mechanician of the Davy Faraday Lalioratory of the Boyal Institution, 
Mr. Haery Payne, who also rendered valuable aid in getting the wliole pressure 
plant with its numerous joints into good working condition. 
