394 ' Transactions of the Society. 



paper, " On Tulley's Thick Aplanatic Object-glass," before the 

 Eoyal Institution * in 1826. Dr. Wollaston states, in 182^^, that 

 with his microscopic doublet he had seen the finest stria? and 

 serratures upon the scales of the Lepisma and Fodura. ft appears, 

 then, that the Podura test was known in 1828, and unknown in 

 1826, therefore its discovery may well be assigned to the year 

 1827. 



Pritchard tells us that the Podura test was discovered " by 

 the late Thomas Carpenter, Esq., of Tottenham, while making some 

 experiments with a plano-convex jewel lens, employed as the 

 objective of an Engiscope, having an Huyghenian eye-piece." 



Dr. Brewster, in 1830, states that the lines on the scales of 

 Podura spring-tail are the most difficult of all the test objects. 



E. J. Quekett, in 1843, says that the most delicate and closest 

 markings on the scale of Podura (4(jiVdo ^°- asunder) may be 

 magnified to 11(^0 diameters, and still be perfectly defined by 

 Mr. Ross' new ^ of 80" aperture. His brother, John Quekett, 

 states that at the time of the pubKcation of Pritchard's " Micro- 

 scopic Cabinet " (1832) nothing but longitudinal and oblique lines 

 could be made out, but that now (1848), with a power of 1250 

 diameters, the scales may be seen to stand out boldly from the 

 surface. At the upper part of the scale they also project beyond 

 the edge. 



Before proceeding, it may be of interest to state that old non- 

 achromatic Microscopes of good construction fail to show the 

 exclamation marks, which can, however, be just glimpsed with a 

 Wollaston doublet, and also with a Lister-Tulley achromatic 

 Microscope — the doublet giving the better image. 



It is not easy to understand what Quekett means by " longi- 

 tudinal and oblique lines," for the unresolved appearance of a 

 Podura scale is like " watered silk." If any one wants to know 

 what the best Microscopes of 1830 were capable of showing, let 

 them place a Zeiss a a, or a Leitz No. 2, on the nose-piece, and a 

 strongly marked Podura scale on the stage, and they will see the 

 " watered silk " just breaking up into exclamation marks (23,000 

 per inch), which is all, or perhaps a trifle more than could be 

 accomplished in 1830. During the next ten years great improve- 

 ments were made in the construction of achromatic objectives 

 by Mr. Lister, who coached both Andrew Boss and J. Smith, 

 and through them Hugh Powell. An inch of Powell's of 1840 

 shows the exclamation marks better than any old lens of that 

 power, and, by reason of its slightly larger aperture, better than the 

 two semi-apochromats mentioned above. 



This description and plate of John Quekett's did duty for some 

 fourteen years, until Eichard Beck's paper and excellent drawings 



* Journ. Roy. Inst., xxii. ser. 1 (1827) p. 265. 



