A NEAV INTERFERENTIAL FORM (3F ELASTICITY APPARATUS. 
163 
Conclusion. 
The advantages which the apparatus now descril)ed appears to the author to 
possess over that of Koch, with which, however, most excellent work has been done, 
are as follows :— 
(I.) Strictly monochromatic C-hydrogen light is employed, instead of the composite 
light corresponding to the two sodium D-lines, which are affected by mutual inter¬ 
ference. Moreover, the use of auto-collimation ensures greater intensity and 
parallelism of the illuminating and reflected rays, and consequently greater perfection 
of interference. 
(2.) The transmitter renders the oljserver independent of the nature oi the surface 
of the plate of the substance Investigated, and all necessity in the case of non- 
polishable and badly reflecting suljstances fn- the doubtful device of cementing a 
small glass jflate to it is eliminated. Further, the troublesome alteration in the 
configuration of the bands, due to the plane surface of the plate being changed to a 
curved one on bending, or to increase of curvature if already more or less initially 
bent, is entirely avoided by the use of the transmitter, and bands of maximum 
excellence, suitable fci' adjustment between a pair of parallel micrometric spider-lines, 
are always obtained, owing to the same selected surfaces being employed to reflect 
the interfering light. 
(3.) The employment of the sensitive method of a modified chemical balance for 
the application of the bending weight aflbrds obvious advantages, not least among 
them being greater compactness of the whole apparatus, which is entirely above the 
supporting table, instead of being partly above and partly below. 
(4.) The mode of controlling the application of the weight, and of regulating the 
rapidity of motion of the bands, is much more convenient and accessible. 
(5.) The measuring microscope is entirely reserved for the determination of the 
dimensions of the plate and its setting, instead of also having to be employed for the 
totally different function of observing the bands. Also, increased accuracy is attained 
in the two rectilinear measuring movements by the provision of a novel method of 
eliminating backlash in the mounting of tlie measuring screws. 
(6.) The error due to movement of the blocks supporting the plate, under the 
bending weight, with respect to the second reflecting surface concerned in tlie 
interference, is eliminated, together v'ith all necessity for the application of a 
correction for it; the glass disc furnishing this surface, and the knife-edges against 
whicli the plate is lient, have a common rigid support, instead of being mounted 
independently. 
The author desires to express his thanks to the Government Grant Committee for 
their grant to defray tlie cost of the instrument. Also to record his sense of 
indebtedness to Mr. J. Skinner, of the firm of Messrs. Troughton and Simms, for 
invaluable help in designing and perfecting the instrument. 
Y 2 
r/6CT.1903 
