RECENT PROGRESS IN CRYSTALLOGRAPHY 101 



passing through the little table of the same metal. These 

 are so arranged that the two reflecting surfaces of glass and 

 either crystal or aluminium are only separated by a thin film 

 of air, and have the necessary minute tilt out of strict parallelism 

 to produce interference bands of the desired width. The lower 

 half of the porcelain tube, together with the chamber suspended 

 below it containing the interference tripod, is immersed in a 

 double air bath which can be heated to the required temperature, 

 during which operation the movement of the bands is observed. 

 For every band which passes the reference mark in the centre 

 of the field the air film at that spot has become altered in 

 thickness, owing to the differential expansion of screws and 

 crystal (and aluminium, if used), by an amount which is equal 



Fig. 4. 



to half the wave-length of the red hydrogen light employed. 

 Hence the half wave-length of light is the grosser unit of the 

 scale, and as the one-hundredth of the distance between two 

 bands can be measured with accuracy by the micrometer of 

 the eyepiece, the measurements can be carried to the three- 

 hundred-thousandth of a millimetre, or the eight-millionth part 

 of an inch. With this instrument the thermal expansions of 

 the sulphates of the alkalis have been carried out. 



The elasmometer, which has since been devised by the writer 

 for the determination of the elastic constants, is shown in 

 fig- 5> without the interferometer. It is designed to measure 

 the amount of bending suffered by a thin plate of the substance 

 investigated, when pressed up into contact, near its ends, against 

 a pair of platinum-iridium knife-edges, by a known weight 



