56 THE MICROSCOPIST. 



properly prepared transparent objects. A good half-inch 

 objective should show the transverse markings between 

 the longitudinal ribs on the scales of the Hipparchia 

 janira, butterfly (Plate I, Fig. 27), and the one-fourth or 

 one-fifth should exhibit markings like exclamation points 

 on the smaller scales of Podura plumbea (Plate I, Fig. 28) 

 or Ijepidocyrtis. 



High power objectives are chiefly used for the most 

 delicate and refined investigations of structure, and are 

 not so suitable for general work. It is with these glasses 

 that angular aperture is so necessary to bring out striae, 

 and dots, and other delicate structures, under oblique 

 illumination. For these glasses, the best tests are the 

 siliceous envelopes of diatoms, as the Pleurosigma angu- 

 latum, Surirella gemma, G-rammataphora subtilissima ; or 

 the wonderful plates of glass artificially ruled by M. Ro- 

 bert, and known as Nobert's test. 



The latter test is a series of lines in bands, the distance 

 between the lines decreasing in each band, until their 

 existence becomes a matter of faith rather than of sight, 

 since no glass has ever revealed the most difficult of them. 

 The test plate has nineteen bands, and their lines are 

 ruled at the following distances: Band 1, y^^th of a 

 Paris line (to an English inch as .088 to 1.000, or as 11 to 

 125). Band 2, T^O^- Band 3, s^th. Band 5, 

 Band 9, I0 '^th. Band 13, ^th. Band 17, 

 Band 19, TI ,i^th. 



It is said that Hartnack's immersion system ~No. 10 

 and oblique light has resolved the lines in the 15th band, 

 in which the distance of lines is about ^ T J^th of an inch. 



The surface markings of minute diatoms are also ex- 

 cessively fine. Those of Pleurosigma formosum. being from 

 20 to 32 in y^^th of an inch ; of P. hippocampus and P. 

 attenuatum about 40 ; P. angulatum 46 to 52 ; Navicula 

 rhomboides 60 to 111 ; and Amphipleura pelludda 120 to 

 130. * This latter has been variously estimated at 100,000 



