334 



KNOWLEDGE. 



September. 1910. 



down : or, the object being on the shp instead of 

 in optical contact with the cover glass, uhen the 

 objective acts, as Mr. Nelson well reniarked 

 in" one of the discussions, rather worse than a 

 dry one That the best results cannot be obtained 

 under such circumstances, the writer trusts the 

 photo-micrographs reproduced here to illustrate this 

 article, will prove. 



papers have been read, more discussions raised 

 upon its structure, than on any other six micro- 

 scopical subjects together. After occupying this 

 position of honour so long, a pit>- indeed would 

 it be \\ere it now deposed. 



Mr E. M. Nelson, in his very interesting paper, 

 published, with a plate of illustrations, in T/ie 

 lounnil of the Roval Microscopical Society for 1907, 



Figure 3. 

 t 1750 diameters with a 2 m.m. apochromatie of Zeiss 



Part of a Podma Scale, first taken a Hiameters 



of 1.40 N.A., and turtlier enlarged to 3500 aiameters 



The interest in the structure of the Podura scale, 

 as a test object, dates back almost to the time xvhen 

 the microscope objective first became achromatised ; 

 that is, effectively so. The attempts at achromatismg 

 began from 1774, but the value of aperture not 

 being understood then, nor indeed until Dr. Goring 

 first called attention to it in about 1827, the)- were 

 all more or less abortive. The first achromatic 

 objective of English construction was made by 

 Tullv, of Islington; some three years after this 



scale was being used as a 



test object. As to 



the scale itself it may s; 



safely be saitl that more 



summarises the theories of different observers on the 



structure, thus : — 



1 —Watered Silk, T. Carpenter, 1S27. 



2.— Wedge-shaped Spines. Dr. Goring, 1830. 



3._Featherlets, John Quekett, 1848. 



4.— Indian Clubs, R. Beck, 1865. 



5.— Beads, Dr. Pigott, 1869. 



6.— Knobbed Heads, F. H. Wenhani, 



7_ Pins, T. F. Smith, 1888. 



8.— Cuneiform. K. M. Nelson. 1907. 



To which mav be added, 



9.— Perforations, Alfred Letheiby, 1H95. 



1870. 



