566 PROCEEDINGS OF THE SOCIETY. 



suggestions of Dr. Clay, the process can be carried out, although the 

 conditions for taking accurate measurements require a more stable 

 support than an ordinary table. The whole apparatus is here shown in 

 use sufficiently rigid to demonstrate the procedure. 



It will be realized that if an instrument can be devised in which the 

 motion of some part of it can be made to show some optical change 

 every time it has moved through a distance of one or one-half wave- 

 length, then by counting how many times this optical change takes 

 place as it moves along, the distance that it has travelled is determined. 

 An interferometer is an apparatus which by means of the interference of 

 light causes a change from brightness to darkness to take place every 

 time one of its mirrors is moved along a distance of half a wave-length. 



If this mirror be fixed to a carriage which also holds the micro- 

 meter to be measured, and the micrometer is' so held that it can at the 

 same time be viewed by a high-power Microscope with a cross line in 

 the eye-piece, then the number of optical changes that take place while 

 two consecutive ruled lines pass the cross-line in the Microscope eye- 

 piece may be counted, and the distance apart of these two lines is 

 obtained as a direct measurement in wave-lengths. Apart from other 

 limitations as to the distance within which the interferometer will show 

 the optical change, the number of units to be counted in measuring as 

 great a length as a metre or a yard is so great that it is not a possible 

 achievement, and a step-by-step indirect method of great complexity has 

 to be employed ; but for the determination of the small distances 

 represented by the rulings of a micrometer this difficulty does not arise, 

 and a direct count can be made with ease. The apparatus being set up, 

 one observer counts the optical changes, while the other, looking 

 through the Microscope, indicates the time when the lines which are 

 being measured cross the cross-wires in the Microscope. 



The interferometer consists of a series of mirrors which receive a 

 beam of light from a radiant source, and divide it into two beams of 

 light, which are afterwards re-combined and viewed by a telescope. All 

 the portions of the instrument are fixed, except one reflecting mirror, 

 which reflects one-half of the divided beam of light. This can be set in 

 such a position that the two half-beams of light, during their period of 

 division and before they re-combine into one, have travelled exactly the 

 same distance. A movement of the reflecting mirror can then be made, 

 so that one half-beam of light has travelled half a wave-length farther 

 than the other ; and when that is the case, interference will take place, 

 and the light will be extinguished. As the mirror is moved farther, the 

 light will again appear, until the path is 1| wave-lengths different, 

 when a second interference is produced ; this goes on at each successive 

 motion of the mirror though the same distance, and the wave-lengths 

 are counted (fig. <!2, p. 539). 



As a matter of practice, it will be seen, on looking through the tele- 

 scope, that the effect is not a complete change from brightness to dark- 

 ness, because this only takes place in the exact centre of the field, and a 

 series of bright and dark bands are seen in the field, which travel from 

 right to left, and are counted as they cross the centre of the field against 

 a line in the telescope. 



I am here showing the apparatus with a Bunsen burner and a sodium 

 flame as being a sufficiently monochromatic light for demonstration ; but 



