336 O.N. Rood on Markings of Pleurosigma Angulatum. 
Without taking any particular interest in the controversy, I 
have devoted some time to photographing this test object, and 
have lately succeeded in obtaining, by the method indicated in 
my article in the September No. of this Journal, a fine, intense, 
sharp negative, magnified 1000 diameters. 
most the whole shell was nearly in focus, a large portion 
being in very accurate focus. When this negative was exam- 
ined by the microscope, the dots were seen to be distinctly cir- 
cular, and well-defined; by the aid of photography an enlarged 
and sharp negative was obtained, (magnified 7000 diameters) 
est yielded good prints on. paper, showing the markings to be 
circular. 
The dots in these negatives were not uniformly shaded but 
presented an appearance like that seen in the diagram, fig. 1. 
Furthermore, although the interior dark circle seemed to 1, 
the eye uniformly dark, this was not actually the case, as 
was demonstrated by copying the negative with the ca- 
mera and varying the time of exposure, when it became 
evident that the deposit of silver grew regularly denser from the. 
2 
circumference to the centre of these little circles; thus 
in fig. 2. 
Mr. Whenam states, that he has ascertained by a ;';, that the 
markings on this test object are to spherical particles of 
d favor this 
The following is an instructive experiment: in a plate © 
brass a large number of circular holes are drilled, their displ 
in question; the plate is then carefully blackened. In mY he 
and 
the photographic camera leveled at it, a distance being selected 
such that small images of the circular holes are obtained. The 
full aperture of the lens should be employed. : 
By varying the focal adjustment of the camera slightly, thes? 
true circles become converted into tolerable hexagons, rhombs, 
or triangles, the latter being less distinctly marked than the two 
