398 Transactions of the Society. 



If the exclamation marks are holes there would be nothing 

 at those points to touch the lower side of the cover, and conse- 

 quently, the light would be totally reflected, and the image must 

 be bright. Again, if those parts where the fluid has run in 

 are, as Mr. Letherby suggests, portions where the upper surface 

 of the scale has been abraded, then the image cannot be black. 

 The inky blackness which is seen can only be caused by the fluid 

 neutralising the total reflection at the lower surface of the cover- 

 glass. The authors of the Micrographic Dictionary say " the scales 

 often become partially or entirely covered with the oily matter, 

 producing an appearance as if the upper layer of the scale were 

 removed, and rendering the markings so pale and indistinct, as to 

 be apparently absent." Mr. R. Beck says that " the markings 

 almost entirely vanish when the irregularities of the surface are 

 removed by the presence of moisture. Fig. 3 represents this 

 appearance on the Podura scale ; and I cannot but think that 

 Dr. Carpenter is mistaken when he says that such scales have lost 

 a portion of their superficial layer by some accidental injury." 



In the face, then, of these facts, the perforation theory cannot 

 be maintained. [In this connection there is one important point of 

 great interest to the brass and glass section of microscopists which, so 

 far as I am aware, has hitherto escaped notice. The " Black Dot " 

 of the vertical illuminator is the " White Dot " of transmitted light, 

 and both are precisely at the same focus. The " White Dot " of 

 the vertical illuminator is below the " Black Dot," and so illumina- 

 tion by the vertical illuminator may be said to be reciprocal to 

 that by transmitted light.] 



Mr. R. Beck in 1862 examined the scales, when mounted on 

 the slip without cover, by reflected light, and gives an interesting 

 account with figures of the experiment. He states that " when 

 the markings are at right angles to the direction of the light, the 

 side farthest off is illuminated ; when they lie in the same direction 

 as. the light, with their narrow ends pointing to it, the broad ends 

 appear like brilliant spots, but when this direction is reversed, the 

 light from the points is so slight that the scales appear to have lost 

 their markings altogether. Now, if the object were an opaque 

 substance, this result would have been a convincing proof that the 

 markings were depressions ; but as we know it to be transparent, 

 it follows that these particular appearances can only be produced 

 by elevations." 



With this I entirely agree. Let me urge every microscopist 

 to examine this experiment for himself. Take a wide-angled oil- 

 immersion condenser (Powell's truncated does very well) and any 

 dry objective from IST.A. • 45 upwards will do. The scale must be 

 on the slip, but in almost every slide a few scales will be found 

 on the slip. Focus the edge of the flame sharply on the scale, and 

 tm-n on the slot so that the scale may be strongly illuminated on a 



