400 Transactions of the Society. 



tion mark in the next row above springs from one arm of the V at 

 the head of the exclamation mark immediately below, which 

 accounts for the exclamation marks not running in a line, because 

 the tail springs from one of the arms of the V and not from the centre 

 of the head. The " Indian club " shaped exclamation mark, which 

 is the accepted image usually figured, shows, as Mr. Smith has 

 pointed out, that, while the illuminating cone is truly centred, its 

 W.A. is too small. With a large axial cone not a trace of beads, 

 or of " the varicose appearance," can be seen upon the exclama- 

 tion marks themselves. 



Passing now from the exclamation marks to the intervening 

 spaces, a peculiar structure will be found, which for want of a 

 better name, may be called a mycelioid structure, because it looks 

 as if the membrane of the scale had been covered over with a white 

 cobweb form of mycelium. The arms of the V's form the 

 prominent, and readily seen, part of this image. The arm of the 

 V which is not joined to the tail of another exclamation mark runs 

 into a more or less vertical line about the middle of the interspace. 

 Thus far the image is not difficult, but in addition to this there are 

 some minute horizontal filaments of great tenuity joining these 

 vertical lines, the visibility of which, by transmitted light, requires 

 a fine objective, and skilful manipulation. 



These horizontal bars are not very close together ; they may 

 count as fine as 40,000 to the inch, but the structure is not as 

 regular as the usual tracery in a diatom, the screw micrometer 

 giving measurements varying from 30,000 to 50,000 per inch. 



As a rule, there are about five or six bars to the length of one 

 exclamation mark. 



When these bars are illuminated by oblique light from a 

 vertical illuminator, they look like beads ; these beads are confined 

 solely to the interspaces, and not the slightest appearance of 

 beading can be seen on the exclamation marks. This beading is a 

 false image caused by the oblique illumination of the bars, but the 

 exclamation marks are entities. This is the reverse of Dr. Pigott's 

 view of the subject, for he called the exclamation marks spurious 

 and the beads true structure. 



[In an image such as this an apochromat runs away and leaves 

 a semi-apochromat far behind ; but in the general run of objects, 

 when a suitable screen is used, there is not much to choose between 

 the pictures rendered by either class of lens, if good.] It will be 

 noticed that this new image of the Podura scale is a white dot 

 picture ; it has been previously pointed out that when a structure 

 is excessively minute a black dot image is unattainable, so we 

 must be content with what we can get. 



In conclusion, it has been seen that these exclamation marks 

 are neither featherlets, nor beads, nor perforations ; so much 

 for what they are not : but what are they ? They are probably 



