144 Mr. A. E. Tutton [May 2, 



You will have gathered that for the prosecution of this work it 

 has been necessary to prepare some hundreds of 60°-prisms and 

 parallel- sided section-plates, all accurately cut to the desired orien- 

 tation with respect to the crystal faces. For this purpose it has been 

 found necessary to have the delicate apparatus constructed which 

 you see before you on the table, and a photograph of which is 

 also thrown on the screen. The crystal, held in a grip-holder, is 

 suspended from a refined apparatus which serves not only for the 

 adjustment of a zone of the crystal's faces to the vertical axis of the 

 instrument, as determined by the observer through the telescope of 

 the collimator signal-slit, but also, as the movements are graduated, 

 for its setting to any position with respect to the axis. Separate and 

 interchangeable cutting and grinding gear are provided, and also a 

 delicate means of varying the pressure of the crystal on the grinding 

 disc, so that the most fragile crystals can be manipulated without 

 danger of fracturing them. 



It is not too much to say of this instrument, that without it the 

 work described to you this evening would never have been possible. 



Another original instrument which you see before you is an 

 apparatus for producing spectrum monochromatic light of any wave- 

 length whatsoever. For all the optical researches have to be carried 

 out in pure monochromatic light, that is, for a series of colours of 

 light, each of which is composed of vibrations of as nearly one wave- 

 length as possible ; for all the optical constants vary considerably 

 for different wave-lengths of light. It is essentially a spectroscope 

 constructed to transmit as large a proportion of tJie light as possible 

 which streams from the condenser of an electric lantern ; a broad 

 spectrum is produced by a highly refractive prism, and is focussed 

 on the back of a second slit, which permits only a selected line of 

 the spectrum to escape, in the same manner as in the well-known 

 apparatus of Sir Wm. Abney. By rotation of the prism the spectrum 

 is moved over the exit slit, so as to permit any desired colour to 

 escape, whose wave-length is known from the reading of the cali- 

 brated circle on which the prism is mounted. 



The apparatus is placed before you just as it is arranged, in front 

 of the goniometer-spectrometer, when determining refractive indices 

 for a series of different wave-lengths. 



There will now be introduced to you another means which we 

 possess of determining the position and shape of the ellipsoid. In 

 an ellipsoid having three unequal rectangular axes, it will be evident 

 that if we consider the elliptical section containing the maximum and 

 minimum axes, there must be a point somewliere on each elliptical 

 quadrant where the radius vector will bo equal to the intermediate 

 axis. Such are the four points C on the figure projected on the 

 screen. The two sections of the ellipsoid which contain these points 

 and the intermediate axis will consequently be circles, and rays of 

 light which pass through the crystal at right angles to these sections 

 will be able to vibrate with equal velocity in all directions perpen- 



