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On some New Appearances in Podura Scale. 



By T. F. Smith. 



(Taken, as read, January 27t7i, 1888.J 



I beg to call the attention of the members of the Club, to a new 

 appearance of the Podura which I do not think has yet been re- 

 corded. On searching over a slide of this scale the other day with 

 an oil immersion, to see if I could not produce Dr. Pigott's beading, 

 I was struck with one that displayed entirely new features. You 

 all know the optician's appearance of this scale, with the exclama- 

 tion marks, blue or red, according to the corrections of the glass, 

 and with a light streak in the middle, more or less extended as 

 the aperture is larger or smaller. But in the specimen before me 

 the usual markings had vanished, and in their stead the whole 

 scale was studded with very slender spines with round heads, and 

 the pointed ends stuck into the scale like a lot of pins stuck loosely 

 and anyhow into a paper, and instead of being blue or red were a 

 pure white. At first I thought there were two sides to the scale, 

 and this was the wrong one, but I soon discovered that this scale 

 was tight against the cover, and that all the scales so placed had 

 the same appearance. 



Since then I have examined many scales on several slides, and 

 am now strongly of opinion that the notes of exclamation markings 

 are spurious, and that the light streak is the true appearance, 

 which has hitherto been seen with the darker outline on each from 

 taking too deep a focus. 



It is a well-known fact that an oil immersion objective works 

 only with its full aperture, when an object mounted dry is well on 

 the cover, and this in itself should be sufficient evidence that the 

 appearance the object presents, under these circumstances, is the 

 truer one. Then, again, the pin-like looking spines are not more 

 than half the diameter of the exclamation marks, and the image is 

 always at its smallest when in focus ; never larger. Another fact 

 which guides me in my estimation of the structure is the observa- 

 tion of a hair with small projecting spines. Here was structure of 



