THE AID OF THE ACHROMATIC FRINGES. 



89 



screws of the mirrors after March 16 were relieved from strain as far as pos- 

 sible, an operation which required several subsequent trials, after which the 

 stability of adjustment (persistently coincident slit-images) was much im- 

 proved. The (internal) temperature (degrees centigrade) of the laboratory 



8 1Z 16 2.0 24 



has also been added, here laid off positively upward. In May and June the 

 external temperature, as reported by the Weather Bureau in degrees Fahr- 

 enheit, is inscribed, and the precipitation in inches (marked K) is often 

 indicated. 



64 ^ * --* t 



ik'l 6 10 14- 18 



If we examine the curves as a whole, the marked variability of the amount 

 of inclination, a (seconds of arc), in March, April, and May, is in contrast 

 with the relative quiescence in the latter part of May, in June and July. 



