260 On the Advancing Aplanatic Power [Monthly Microscopy 
power is here reduced at least ten times by changing from the 
highest eye-piece and long tube to the lowest with a closed tube ; * 
and that at the edge of flexure I have frequently noticed a ribbed 
appearance corresponding to rouleaus of beading on the surface. 
The history of our instrument, not without cause for humilia- 
tion, presents significant warnings for present and future devotees. 
In the days of Quekett, a name dear to every English microscopist, 
we still remember the admiration excited by the grand chef-d’ceuvre 
of his plates—the chequers, lines and squares of the diatoms so 
splendidly displayed. The influence of this kind of portraiture, 
false though it is now known to be, is still felt. Many a costly 
glass is still in use only capable of producing these elegant illusions. 
T have seen a 7th from which I could never educe anything but 
“oblong squares” black and white from the Formosum. The 
errors to manceuvre this delusion must have been considerable. 
Again, when our distinguished President described the “ plate of 
marbles,” and the spectator exclaiming, The microscope makes a new 
start on the Queen’s birthday, 1869 ... . he added, “the terms 
‘striation and lineation’ are no longer admissible... . ‘striz 
and lines’ in the books we now have in our hands are just as little 
applicable to the rows of hemispheres on the surface of a diatom- 
valve, as they would be to a hayfield with its rows of haycocks. 
.... The markings of these diatoms are described and figured as 
hexagons .. .. and Puorocrapny stands by as an attesting 
witness.” 
“Even a 2rds inch by Wray, with the D and double D eye- 
piece, shows the ‘ plate of marbles’ on P. Formoswm, with abundant 
light and perfect achromatism.” 
Our President brought out in this paper, most forcibly, the 
advantages he had derived from the substitution of parallel rays :— 
“When they (the lines) reach their proper place, the hemispheres 
seem to start into existence, and the degree of elevation is conferred 
per saltum..... There they are. I can number them, weigh 
* Power of J, with Ceye-piece.. .. .. .. «. ‘2,500 
‘3 ; LONE HABE rise)! ele! fase) Lew et tiie 
<3 2 long tube and DD eye-piece .. 11,000 
‘ m short body and lowest eye-piece 1,100, or ten times less, 
nearly. + P. 82. 
