MICKOSCOPIC TEST OBJECTS. 
149 
EXPLANATION OP PLATE LYIII. 
BY DR. ROYSTON-PIGOTT, M.A., F.RA.S., P.R.M.S. 
Late FeLlow oe St. Peter’s College, Cambridge. 
T HE accompanying diagrams Lave been delineated under the 
magnifying power of a half-inch and one-sixteenth Powell 
and Lealand immersion lens,* the only kind of objective which 
has resolved Nobert’s nineteenth band of lines drawn at about 
112,000 per inch. The power varies from 500 to 5,000 dia- 
meters. Most of the appearances have been exhibited to Mr. 
Reade, at whose request the drawings have been made, who 
will readily recall one of the novel Poclura appearances, No. 4, 
a valuable preliminary definition, as one among the manifold 
changes which this precious object assumes, according to the 
light, quality, and corrections of the microscope. The double 
black interference lines on the provisional ribbing, as repre- 
sented by fig. 9, is another astonishing effect, occasionally also to 
be seen with the striated beading shown at figs. 7, 8 ; whilst 
fig. 6 is a particularly striking result of purposely changing the 
spherical aberration of the objective. Fig. 7 represents the 
lower stratum of beading seen with the immersion one-sixteenth 
and oblique light. The upper rows are only to be seen as aided 
by a very minute brilliant achromatic pencil of illuminating 
rays : 8 gives both headings at once. Fig. 5 represents the 
false standard appearance of the Podura test, accredited for 
many years by microscopists. 
Diatoms . — Fig. 1 is a faint resemblance to the form of the 
resplendent beading of the highly refracting silicious substance 
of the Pleurosigma formosum , which ranges about 32,000 to 
the inch, with an immersion power of 5,000 diameters. Cres- 
cent shadows are sharply displayed, and the beading, glowing 
with a deep ruby tint, is crowned with tips of golden light. In 
the interstitial spaces fine traces of a brilliant blue colour, lost 
in the azure brightness, may be observed of a secondary struc- 
ture, resembling spherules. But at present they appear too 
spectral to be definitely discussed ; but they form a most vivid 
contrast to their ruby companions in the field. It would seem 
very little light is lost by transmission. In proportion as the 
definition becomes finer the light appears to increase, even with 
the same source of light and reduced pencil. 
Large isolated beads, scattered by twos and threes out of the 
* Nearly one- twentieth of an inch focal length, with the adapted water 
lens. 
