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THE POPULAR SCIENCE MONTHLY. 



versy which is not yet concluded. Men equally eminent have taken 

 opi>osite sides and expressed the most contrary opinions ; and I now 

 2)ropose to give a brief rhume of what has been said and done in 

 regard to this subject, because the matter is full of instruction to those 

 interested in microscopical research. Not that the markings of the 

 podura are of the slightest importance, or have any scientific signifi- 

 cance, but the gravity of the conclusions which are sought hinges upon 

 the fact that, if the views of Dr. Piggott are correct, our most emi- 

 nent microscopists have been promulgating false and erroneous state- 

 ments respecting the form of a well-known and common object ; and, in 

 whatever light the controversy is viewed, the humiliating confession 

 must be made that they are still unable to determine the correct focus 

 or the proper method of illuminating it. 



Di-. Piggott commences by calling resolving the podura-scale " a 

 difiicult enterprise," and then describes the beaded appearance in the 

 following manner: "Under a low power, as 80 or 100, the podura-scale 

 is remarkable for its wavy markings, compared to watered silk ; rais- 

 ing the power to 200 or 250, and using a side-light, the waviness dis- 

 appears, and in its place longitudinal ribbing appears ; with 1,200, they 

 divide themselves into a string of longitudinal beads ; but with 2,300 

 they appear to lie in the same plane and terminate abruptly on the 

 basic membrane ; in focusing for the beads attached to the lower side, 

 the beadings appear in the intercostal spaces." 







U 



7^ 



Fig. 4. The same Poduea-Scale as viewed under Diffeeent Phases of Oblique Light. 



(Westropp.) 



Respecting the old received views of the podura-scale, Dr. Piggott 

 says : " With 300 to 500, the celebrated ' spines ' appear, according to 

 the size of the scale, as very dark tapering marks (like 'notes of admi- 

 ration' without the dots ' ' '). To see these clearly with 2,500 has 

 been considered the ne plus ultra of microscopical triumphs, and it is 

 consequently with no small diffidence that the writer ventures to trav- 

 erse the belief of twenty-five years." 



Dr. Piggott further states that he reckons these beads to be g p^o ^ 

 * 1 g 0*0 o^ ^^ \w^ in diameter, and that the " spines," which he 



160000 



calls spurious, really embrace in general three or four beads, while the 

 intervening space abounds with beads seen through the basic mem- 

 brane, and very difficult of observation without special management ; 



