RECENT PROGRESS IN CRYSTALLOGRAPHY 99 



broad spectrum is produced by a very large and exceptionally 

 highly dispersive, although perfectly colourless, prism, and is 

 focussed on the back of the second slit, which permits only 

 a selected line of the spectrum to escape, in the same manner 

 as in Sir William Abney's well-known apparatus. By rotation 

 of the prism on a delicately divided circle, which is calibrated 

 for wave-lengths, the spectrum is moved over the exit slit, so 

 as to cause any desired colour to escape, whose wave-length 

 is given by the circle. The line of issuing light is suitably 

 diffused by means of a screen of very finely ground glass, 

 carried as an attachment in front of the exit slit. In the 

 illustration the goniometer-spectrometer is shown in position 

 before this screen, as when refractive indices are being deter- 

 mined. 



For the determination of the thermal expansions the inter- 

 ference dilatometer shown in fig. 3 was devised. It is as 

 equally applicable to the measurement of the thermal expansion 

 of any small bodies as to that of crystals, and depends on 

 Fizeau's principle of measuring the displacement of interference 

 bands. The optical apparatus, the interferometer, is indeed 

 common both to the dilatometer and to the elasmometer, the 

 writer's elasticity apparatus. The light from a hydrogen 

 Geissler tube is made to traverse an auto-collimation telescope, 

 which directs it horizontally to a train of prisms carried at the 

 head of a separately supported vertical tube largely constructed 

 of porcelain. The red C-light is here selected and directed down 

 on to two surfaces designed to reflect it in such a manner as 

 to produce interference bands. The latter are visible in the 

 eyepiece of the telescope owing to the reflected light being 

 caused to retrace its steps with just sufficient lateral displace- 

 ment to enable it to pass through the open half of the auto- 

 collimation aperture, instead of striking the little totally re- 

 flecting prism whence it originally proceeded which closes the 

 other half. The crystal block is laid on a small table of 

 platinum-iridium, as shown in fig. 4, and its upper surface, 

 if polishable, forms the lower of the two reflecting surfaces ; 

 if unpolishable, a small disc of aluminium is laid over it, whose 

 upper surface furnishes the desired reflection with exactly 

 the right intensity. The upper reflecting surface is the 

 lower surface of a glass plate (slightly wedge-shaped), 

 which rests on a tripod of three platinum-iridium screws 



