302 
Transactions of the 
TMonthly Microscopical 
L Journal, Dec. 1, 1869. 
membrane; whilst on the other hand deepening the objective to 
gain power hmits the focal depth or penetration ; the amount of 
this depth even of a |th being exceedingly small. 
By estimation comparing these beads with those of the P. 
formosum of 3 o-io o inch in diameter, the observed Podura beads 
may be reckoned at 5-0^0-oth to t5 oVoo-th of an inch in diameter. 
The " spines " usually drawn really embrace in general three or 
four beads, whilst the intervening spaces abound with beads seen 
through the basic membrane, and very difficult of observation 
without special management. • 
750 diameters will show with fine definition and a long-drawn 
tube and good penetration, beaded striae upon both sides of the 
scale when coarsely marked. 
1800 with a 4th and nearly direct Hght (which should be 
formed as free from colour and astigmatism as possible) will show 
in favourable cases chains of beads lying upon the upper surface. 
I cannot here too strongly call attention to the beautiful pheno- 
menon, which I have always endeavoured to obtain as a fine and 
reliable test of approaching aplanatism and heralding a fine defini- 
tion, In examining striated bodies — longitudinal bands glisten 
with a ruby tint upon a green or yellowish green ground. The 
bands appear like pellucid semi-transparent cylindrical ribs, and the 
flashing of these bodies with a ruby glow is a signal in my expe- 
rience that the aberration approaches its minimum; when the 
beading dispels the mist and haze always accompanying the spurious 
" spines." 
With a power of 2500 the beading may be seen to terminate 
abruptly, and commence abruptly near the edge of the scale. 
The most difficult definition is that of the substratum of beads 
glimmering through the membrane nearest the light ; on the other 
side they are generally of a very bright yellow-green colour, con- 
trasting prettily with the deep ruby colour of the upper beads. 
Avaihng myself of the aplanatic test above described, and focus- 
sing carefully the upper surface of the following tests, I have had 
the rare pleasure of seeing the manner in which the structures of 
several beautiful objects are arranged. 
A. Battledore scales of azure blue. 
The whole surface is beaded over: the large beads seen 
with a J-inch are formed of a mass of strings of beads 
crossing and recrossing. 
B. Fine transparent and smallest striated scales of azure blue. 
The upper ribs appear as distinct ruby-coloured beading; 
between and beneath which are seen joartly obscured 
longitudinal rows parallel to and immediately behind the 
upper set. 
