Knowledge. 



With which is incorporated H;ird\vicl<c's Science Gossip and the Ilkistrated Scicntitic News. 



A Monthly Record of Science. 



Conducted by Wilfred Mark Webb, F.L.S.. and E. S. Grew, M.A. 



SEPTEMBER, 1910. 



THE MICROSCOPE AND THE PODURA SCALE. 



l'>v T. F. SMITH. 



The interesting note by Mr. R. T. Lewis, on tbe 

 Podiirae, must have proved pleasantly reminiscent to 

 all lovers of the Microscope whose lifetimes, as 



'f^StK 



W 





Figure 1. 



A Podura Scale, presenting the conventional 



appearance of " exclamation marks " with the 



lisht-streak upon each. Taken with a dry 



objective, magnified 1150 times. 



adults, have extended over the same period, w hen the 

 battle on the structure of the scale was raging. Not 

 that it professes to be settled even nou'. Mr. Lewis 

 speaks of the scale as being a common test before the 

 introduction of the high power oil immersion and 

 apochromatic lenses, and rightly so, yet, since 1887 

 there have been, at least a dozen independent 

 communications to the Royal Microscopical Society. 

 on the subject of this scale. They have this 

 peculiarity in common, too, that each observer has 

 a theory of his own to promulgate. The last was 



by that eminent microscopist, Mr. E. M. Nelson, so 

 recentlv as 1907. 



In spite of these labours, it cannot be said that 

 the employment of wide apertured objectives upon 

 the scale have been justified. Indeed, with two or 

 three exceptions, it cannot be said that a wide 

 aperture lias been employed at all. The pages of 

 The Journal of the Royal Microscopical Society w ill 

 bear testimony to this, where, photographicalh', idl 

 that has been attempted is to produce the conven- 

 tional ■' exclamation marks." Now these are 



Figure 2. 



A small Podura Scale, showing secondary 



structure. Taken with a Zeiss 2 ni.m. objective 



of 1.40 N.A.. magnified 1750 times. 



essentially, when an oil immersion is used, either 

 the production of the objective tlirown very 

 much out of adjustment, and much stopping 



Hi 



