071 the Podura Scale. By E. M. Nelson. 397 



views of the eminent microscopists just given. Pigott's beads 

 must, for reasons stated above, be ruled to be quite out of court. 

 But what about featherlets ? They have been seen by J. Quekett, 

 Edmunds, Moore, Morehouse, Hitchcock, Vereker, and Wright. The 

 objections to featherlets are : 1. No one has observed a missing 

 featherlet. 2. They seldom protrude beyond the margin of a 

 fracture ; but very often half an exclamation mark may be seen on 

 one side of a fracture and half on the other. This has been well 

 shown by Dr. Mercer (see his photograph). Personally, I have 

 neither seen nor experimented with Dr. Arnold's electric spark, 

 but notwithstanding, the weight of evidence seems to be over- 

 whelmingly against featherlets. 



The latest suggestion is that of Letherby that they are apertures. 

 One very weighty objection to this view of the question is that the 

 exclamation marks de-polarise light ; for if the scale be rotated in 

 a dark polarised field the exclamation marks become brilliantly illu- 

 minated when a particular azimuth is reached. This was exhibited 

 before the Society many years ago. There is another, and very 

 important point, where, if I read Mr. Letherby's paper correctly, 

 he seems to have got quite off the track. Several very eminent 

 microscopists, who have devoted a great deal of time to the Podura 

 scale, are agreed that these scales contain a kind of fluid (oil ?), and 

 it is common knowledge that these scales stick to the cover-glass 

 by virtue of this fluid, and no mounter, however expert, is able, 

 when once they have adhered to a cover-glass, to remove and 

 remount them. This fluid can be seen in figs. 3 and 6 of 

 Mr. Letherby's photographs ; but Mr. Letherby, who has photo- 

 graphed this object so well, gives a totally different explanation of 

 the phenomenon. He says that this is not fluid, but the abrasion 

 and absence of the upper membrane. It is upon this point then 

 that we will join issue ; but a digression is necessary to remind any 

 who may have forgotten it, that a vertical illuminator (in spite of 

 its absurd name) is an instrument for making an objective illumi- 

 nate its own object, and that its action depends upon the principle 

 that light passing out of the front lens, through the oil and into 

 the cover-glass, at an angle greater than the critical angle, is 

 totally reflected from the lower side of the cover-glass. 



Three results follow in consequence : (1) an object not in 

 optical contact with the lower side of the cover-glass is invisible ; 

 (2) the object, when seen, is seen in the image of the source of 

 light, and, when the object is removed, the image of the source of 

 light remains ; (3) any object which appears black or dark does so 

 because it touches the lower side of the cover-glass and allows the 

 light, which otherwise would be totally reflected, to escape. 



Now, if a Podura scale be examined with the vertical illumi- 

 nator, what do we see ? (1) The exclamation marks are black. 

 (2) Those portions where the fluid has run in are inky black. 



