'j°"rnl'i,'junrri'im'] On lUgh-power Definition. 305 



there is only a faint indication of a core, or median line. In the 

 portion of this same midrib situated in the scale the rows of beads 

 can be made to apjDear. I have therefore no doubt that they are 

 spurious. In fact, in the Forinosum the row of beads next to the 

 midrib are much finer, or about half the size of the others, and a 

 spurious image of these can be thrown within the rib. 



Under a o-Vth the Formosum is a most superb object. The 

 spherules are perfectly isolated, and appear like beads of coral on a 

 deep sky-blue ground, and at the fractured edge they overhang in 

 some places. 



VI. — On a Neiv Critical Standard Measure of the Perfection of 

 nigh-^poiver Definition as afforded hy Diatoms and Noherfs 

 Lines. By Dr. Eoyston-Pigott, M.A., Cantab., Fellow of 

 the Cambridge Philosophical and of the Koyal Astronomical and 

 Microscopical Societies of London ; formerly Fellow of St. Peter's 

 College, Cambridge. 



The study of Diatom- and Nobert's lines unquestionably rewards the 

 ardent observer for years of application and research. By such 

 studies cliiefly microscopy has reached its proud position among the 

 advanced sciences of the nineteenth century. What was deemed 

 impossible ten years ago is now with the microscope a common feat 

 performed at will and at once, as the resolution of Ehomboides, 

 which good observers might formerly be hours in attaining. 



Further advances can only be made by searching out errors yet 

 to be remedied : it is unphilosophical to declare perfection has been 

 reached — as a bar to inquiry. The satisfied optician, in the face of 

 modern improvements, is apt to feel it would be far better to let 

 well alone and discourage further refinements in optical science. Our 

 motto must still be " Onward." 



The great obstacles to minute observation may be summed up in 

 two words : imperfect correction and exaggerated difiraction. 



The former is perhaps insufficiently studied by microscopists, 

 who often purchase their' glasses on trust ; the latter is a subject 

 which remains to be thoroughly investigated and exhausted. Both 

 these causes distort, derange, and disfigure the true definition of 

 minute objects, and especially the appearance of the celebrated lines 

 of Nobert's Test-Plate. 



Diffraction lines are not confined to the images of brilliant ob- 

 jects. If a transparent or rather opalescent and pellucid film of a 

 variegated substance traced with dark spots and lines be examined 

 under a high power, when illuminated by the direct rays of the sun, 



VOL. III. X 



