MICROSCOPIC TEST OBJECTS. 
147 
properly; secondly, that their best object-glasses are afflicted 
with sufficient spherical aberration to have rendered the struc- 
ture which he describes invisible.” — “The Student,” Feb. 1870. 
Dr. Piggott’s views “ On High Power Definition ” were com- 
municated to the Royal Microscopical Society at the end of 
May last, and his paper, read after the summer vacation, was 
published in the December number of the “ Monthly Micro- 
scopical Journal.” 
Mr. Slack states that “ all difficult seeing is in some suspense 
through Dr. Piggott’s researches and it really seems to me 
that microscopical results are now in a transition state. Two 
well-known facts point to this conclusion. 
First, the extraordinary advantage of employing either 
parallel rays, or rays of small convergence in illumination. 
Second, the better correction of spherical aberration. 
Dr. Piggott, like myself, generally employs a single illumi- 
nating pencil, in his case virtually parallel, and therefore I am 
not surprised at his announcement of “ Crescentic shadows ” 
as proving that elevation of diatom-markings which I had de- 
tected myself not a week before his paper was written. There 
are, indeed, many facts and secrets of structure which can be 
revealed by such a beam alone. Light virtually parallel can 
lead to no confusion, but the crossing and recrosdng of an 
infinite number of rays produce such a multiform shadow as to 
de-shadow or obliterate a true light and shade portraiture, 
which is the essence of every picture, and the very soul of 
every natural representation. Happily there is no difficulty in 
obtaining parallel rays of sufficient intrinsic brightness. If in 
any case the parallel simple beam from the equilateral prism 
may be found, as it were, too diluted, the parallel condensed 
beam from the hemispherical prism will be a ready and suffi- 
cient substitute. 
Dr. Piggott has also made a very decided advance in the 
better correction of residuary aberration, a point which has, I 
believe, been almost completely ignored — nay, even denied — 
until recently, by accurate observers as well as by distinguished 
opticians. From my own experience in Dr. Piggott’s studio, I 
have no doubt that his colour test , a most interesting feature in 
his experiments, is the result of his finer balance of the aberra- 
tions. Before I had the pleasure of seeing Dr. Piggott, I had 
an intimation of the “ colour test,” as noticed in my description 
of P. angulatum , but only to the point at which residuary aber- 
ration stopped it. For the very same power, under the influ- 
ence of Dr. Piggott’s correcting lenses, shows intensity of colour, 
and the beads of Degeeria jplumbea appear a brilliant azure 
blue in the deeper focal plane, and of a rosy radiant pink in 
the upper plane. This new fact is perhaps one of the most 
