Note on P. angulatum, t^c. Btj John Ware Ste'phemon. 187 
appearance produced by six equidistant spectra in a circle; these 
correspond precisely with the sjgectra of Angulatum, and the 
drawing deduced from theory, as shown in the annexed figure 
from Dr. Eichhorn’s essay, is in exact accordance with that now 
presented by P. angulatum under the oil-immersion lens; but 
the smaller markings between the circular spots of the drawing 
are indicated by faint points only, under the microscope, without 
any definite shape, and would possibly have escaped observation 
altogether had not mathematical theory pointed out that such 
images ought to appear. The great interest in this drawing is, 
that Dr. Eichhorn had never seen a diatom under the microscope, 
and gave graphically notwithstanding the purely mathematical result. 
If we still further manipulate the spectra by shutting out each 
alternate spectrum, leaving only three, we come back again to the 
hexagonal markings ; but, as I have previously shown, they are 
three times as numerous as under normal conditions, the length of 
the sides of the equilateral triangle formed by the spectra being 
respectively as \/ 3 : 1. Hence we see that we can by regulating 
the spectra show at pleasure large or small hexagonal markings, 
circular spots, or even rectangular figures,! Hie latter form depend- 
ing on the admission of a spectral image of the second order. 
Taking a valve of P.formosum, and simply stopping out the 
centre of the objective as before, one sees on looking down the tube 
without the eye-piece, the whole of the back of the lens filled with 
the spectra arising from the right-angled lines (or dots) of the diatom ; 
but in this case, from the coarseness of the markings, the perfect 
spectra are admitted, and we have a beautiful white object on a 
black ground ; but instead of the “ plate of marbles ” the appearance 
presented is that of circular holes punched out of a silvery plate. 
Treating a test scale of Podura in a like manner, i. e. with 
perfectly central light from the condenser and no dioptric beam, we 
have a silvery scale with parallel black lines running from one end 
to the other, with a total disappearance of the exclamation markings. 
This appearance corresponds to a great extent with that demon- 
strated by Mr. Beck on one of our scientific evenings, on which 
occasion condensed moisture from the breath could be seen running 
down the scale. 
It will be remarked that the black ground produced by stopping 
out the centre of the objective differs essentially from that obtained 
by a stop in the condenser. In one case the light is stopped after 
entering the objective, and in the other the great obliquity of the 
incident ray prevents the lens taking it up. 
* ‘ Monthly Microscopical Journal,’ vol. xvii. p. 88. 
t ‘Journal of Koyal Microscopical Society,’ vol. i. p. 54. 
