102 The Advancing Poivers of Microscopic Definition. 



" P. PEEEGEiNA. — Median baud and costse resolvable into rows of 

 beads. 



" P. LATA. — Median band consists of rows of fine beads at riglit 

 angles to costae. 



" P. DiVERGENS. — Witb D eyc-piece the l^th resolves the median 

 band into rows of beads wbich curve round the expanded ends of the 

 two furrows." 



Colonel Woodward kindly informs me that he has not yet been 

 able to see the median bands of either Formosum, Angulatum, or 

 Ehomboides resolved into beads ; but after this feat by Mr. Slack, I 

 trnst he will be encouraged to produce photographs of the beaded 

 median line of diatoms, notwithstanding they were somewhat con- 

 fidently denounced as spurious after a statement had been made by 

 the writer in the following words :* — 



'' In my hands an immersion 1-1 6th has displayed the beading 

 of which the median ribs of the 



Foniiosum (a) (diameter, tliree to one) 

 Angulatum (6) (diameter, three to two) 

 Bhomhoides (c) „ „ 



appear to be composed. Very curiously the diameter of these beads 

 bore a diflerent proportion to the general beading in each case, as 

 above indicated — a confirmatory fact for the truth of their existence, 

 and negativing the supposition of their being spurious, and guaran- 

 teeing their integrity." 



Tlie Advancing Definition of Lined Objects. 



As it may be laid down as an axiom that in aU optical glasses 

 there is a residuary spherical aberration of either the first, second, 

 or third order, the very finest glass now made bears to the very 

 finest and most delicate test-object somewhat of the same relation 

 that ordinary glasses bear to the rougher tests. 



The ordinary glass showed lines ; the extraordinary, beads. So 

 where a splendid glass now only shows lines as in the Amjyhipleura 

 jpellucida, a glass of transcendent quality will by-and-by resolve 

 this again into its components. 



A Tolles' xVtli immersion, with monochromatic light, has 

 shown the Amjjh. j^ellucida with 87,000 lines to the inch, counted 

 by Mr. Bicknell, of Cambridge, U.S., though mounted in Canada 

 balsam. Messrs. Powell and Lealand show a much finer scale than 

 this, without monochromatic fight, with their rVtli. 



A very great source of difficulty is the spurious set of lines 

 which haunt every fine line shown in the best microscopes, even 

 when the corrections are pushed to the finest point of adjustment. 



* November number of this Journal for 1870. First described in 'Pop. 

 Science Review,' April, 1870, Plate. 



