MICROSCOPIC TEST OBJECTS. 
141 
therefore proposed to obtain the shadows of the lines, not by a 
circle of light, but by three separate small pencils of light of 
proper intensity in the kettledrum, to be placed by the revolu- 
tion of the substage at right angles to the lines to be resolved ; 
and if this were the true structure of the P. angulatum , the 
principle of illumination is correct. The result also appeared 
to be satisfactory, though in reality it only confirmed the 
universal error as to the existence of hexagonal markings, 
formed by the crossing of two equilateral triangles of these 
shadow lines. My first two papers on this subject and on the 
single and double hemispherical condenser are contained in the 
“ Quarterly Journal of Microscopical Science,” July 1861 and 
January 1867. 
Still I had many misgivings as to the presence of hexagons, 
and, after further examination, I inclined rather to the suppo- 
sition that the markings were circular and probably elevated. 
I therefore felt the force of Dr. Carpenter’s dictum, that ct sus- 
pension of the judgment whenever there is room for doubt is 
a lesson which the microscopist cannot too soon learn or too 
constantly practise.” When I was thus shut up in Doubting 
Castle, asking myself the question, “ Is it, or is it not ? ” I very 
fortunately purchased of my friend Mr. Powell, his little hemi- 
spherical condenser for showing the transverse lines of that 
prince of puzzlers, the Amphipleura pellucida. In using this 
lens I saw, upon reflection, that a small portion only was avail- 
able. I therefore covered the whole lens with tinfoil, and then 
cut out opposite apertures on the plain and convex surface, so 
as to obtain in point of fact a small prism-illuminator. This 
answered the purpose well, and it immediately occurred to me 
that the equilateral prism itself might be substituted for the 
prism-slice of Powell’s lens. I tried it, and with what result — * 
may I say with what success ?■ — was communicated to the Royal 
Microscopical Society, in papers published in the “ Monthly 
Microscopical Journal” for July and August 1869. Mr. Slack 
has subsequently published two papers on this subject in the 
ee Student and Intellectual Observer ” deserving the careful 
attention of those who are engaged in these histological studies, 
and Mr. Highley has brought out a short tract on this new 
method of illumination with copious and interesting details. 
THE EQUILATERAL PRISM. 
I saw at once the great advantages of the equilateral prism 
and its single parallel pencil of light. I had no longer, as with 
the kettledrum, two suns in my firmament shining at right 
angles to each other, but one source of proper light properly 
placed ; and therefore, instead of the false appearance of lines 
