Boijal Microsco;pical Society/. ' 247 



The cause of this black shadow is not without interest, and it 

 depends upon the following principle. 



If we observe an isolated spherule of Mr. Slack's new silica 

 slide, some very beautiful and instructive phenomena present 

 themselves. 



(1) Large spherules show a beautiful miniature image of the 

 lamp flame above ; and below it out of focus, a black ring or boimdary 

 of the spherule with a faint diffused light over the spherule and 

 around it. 



(2) Small spherules give a round disk of light above, and below, 

 a dark boundary or circular ring, surrounded with a brightish 

 halo. 



(3) Still smaller spherules give a round disk of light; and at a 

 certain stage the disk remains the same in size, notwithstanding the 

 spherules are taken smaller and smaller. 



Now a black shadow of an oval shape is immediately developed 

 by accurate side illumination of the purest kind, viz. by a low-power 

 object-glass free from aberration. The intensity of these shadows 

 can be brought out by the ordinary mirror, or prism. 



[If you try to get an image of the lamp from a mirror you will 

 find at one place the flame is a white stripe, and at a difierent 

 distance it becomes a larger stripe at right angles to its former 

 position.] 



If a diatom be employed (large in proportion to the poorness 

 of the glass), the black shadow may be seen of a crescentic form ; 

 and by revolving the stage this black crescentic shadow may be 

 made to revolve, so that every part of the spherule is equally 

 capable of producing the same shadow, the finest proof we can pos- 

 sibly conceive of sphericity in a refracting particle. 



If a row of spherules be thus illumiuated and observed feebly, 

 the individual shadows degenerate into a notched dark line; still 

 more feebly (as by an instrument unequal to its task) into a blurred 

 thick dark line. 



[Just the same as we used to see ihe formosuyn and angulatum 

 with the belauded glasses of days gone by.] 



Mr. Slack's slide exhibited thus with the highest delicacy of 

 definition at our command, develops a mass of oval black shadows 

 as intense as ink upon snow, every one produced by its correspond- 

 ing bead. 



The exclamation markings of the Podura disappear at once by 

 this treatment, which could not happen if they were realities. 



They are replacal by long ribs and double black shadows, when 

 viewed with the long axis of the scale placed perpendicular to the 

 direction in which they are illuminated. 



Besides, in the larger sort every one of its component spherules 

 exhibit the same rotating black shadow as Mr. Slack's silica beads 



