A WAVE-LENGTH COMPARATOR FOR STANDARDS OF LENGTH. 3 



glass surface, as it should be if it is an exact copy, (3) to traverse that microscope 

 until it is so adjusted, and (4) to observe and count the number of interference bands 

 which move past the reference spot during the process. The product of this number 

 into the half wave-length of the light used to produce the bands thus obviously 

 affords the difference between the two lengths included between the defining marks 

 on the two bars. 



The simplicity of the method is one of its greatest recommendations. It appears to 

 have been hitherto thought that the mechanical difficulties involved would render 

 such a method impossible to carry out in actual practice. The experience gained by 



Fig. 2. Central part of comparator, showing interferometer. 



the author in connection with the elasmometer, however, had shown that Messrs. 

 Troughton and Simms, its constructors, were able to make a fine screw movement to 

 work so exceedingly slowly, and so absolutely steadily, while at the same time driving 

 a considerable weight, that interference bands produced between a pair of glass 

 surfaces, one of which moved with the slider driven by the screw, moved past the 

 reference spot, as seen in the interferometer telescope, with the required complete 

 steadiness and precision and became arrested instantly when the observer ceased to 

 turn the screw. 



This experience has been applied with perfect success to the new comparator. The 



B 2 



