82 Dr. W. M. Watts on a New Form of Micrometer 



venient line of reference, can be brought into coincidence with 

 the lines to be measured. 



The figure represents the micrometer as applied to a direct- 

 vision spectroscope of Browning, a and b are the two strips 



cut from a lens of about 2 feet focus, placed in front of the com- 

 pound prism contained in the body c. Of these a is fixed ; but 

 b can be made to slide over a by means of a screw d; and the 

 displacement is measured by means of the graduated head e 

 and the counter/. 



In order to reduce the indications of this instrument to wave- 

 lengths, I have constructed a series of interpolation-curves from 

 the data obtained by careful observation of the solar spectrum. 

 One of these curves is for use when the sodium-line (D 1 = 5889) 

 is employed as the reference-line, another when the Fraunhofer 



