306 On a Neiv Critical Standard Measure 



rMonthly Aflcroscoplcal 

 L Journal, June 1, ia70. 



minute dark rings may be observed surroimding each dark point, 

 and with every change of the focns the whole surface appears to be 

 in motion with expanding and contracting dark rings of diffraction. 

 Similarly if the toy-stone called the philosopher's stone enspangled 

 ■with crystallized metallic points formed in melted glass, which are 

 seen principally of hexagon and triangular form (when brilliantly 

 illuminated with a bull's-eye condenser), be examined with a fine jth 

 objective, each crystal appears bordered with symmetrical dark dif- 

 fraction lines varying in number. 



Fig. 1. 



Indeed, whatever shape the bright object be, it is accompanied by 

 these spectral lines or fringes. 



In examining the beads of the Ehomboides I was yesterday sur- 

 prised to see two halves floating asunder as spectral images from the 

 effect of two oblique pencils, and joining together only when the 

 glasses were properly adjusted. 



Fig. 2. 



And in the same manner spectral lines in Nobert's bands may be 

 made to float before the eye of the observer. I have thus frequently 

 seen the blank spaces between Nobert's bands crowded with lines 

 more real in appearance than the true. 



The central ribs or median lines of the Ehomboides' diatom were 

 also observed to be composed of an extensive rouleau of beads in 

 close contact, much closer set than the general beading, just in the 

 same way" as I lately succeeded in distinguishing four rows of beads 

 composing the median lines or ribs of the Pleurosigma Formosum 

 and Angulatum. 



