186 Transactions of the Royal Microscopical Society, 
III . — Note on the Effect produced on P. angulatum and other 
Test Objects hy excluding the Central Dioptric Beam of Light. 
By John Ware Stephenson, F.E.A.S., Treas. K.M.S. 
{Bead hefore the Eoyal Microscopical Society, June 5, 1878.) 
In the hope that it may be of interest to the Fellows of the 
Society, as illustrating, in some measure, the demonstration by Pro- 
fessor Abbe, that the resolution of lined as well as of some other 
objects depends on the diffraction spectra produced by the struc- 
ture of the objects themselves, on the light passing through them. 
I have this evening placed on the table a slide of Pleurosigma 
angulatum under the new large-angled oil-immersion lens with 
the centre stopped out, an experiment which also proves, if that be 
necessary, that in an objective of the best construction the centre 
is not essential to excellence of definition. 
The light passing through the object in the present experiment 
is transmitted by the achromatic condenser, and is absolutely central. 
Under ordinary conditions this would show, on taking out the eye- 
piece and looking down the tube of the instrument, one bright 
central light from the lamp, with the six equidistant surrounding 
diffraction spectra, produced by the lines (if indeed lines they 
be) in the object itself ; but at the back of the objective and close to 
the posterior lens I have placed a stop made of black paper, which 
entirely excludes the central beam of light ; in this stop, however, 
six marginal openings have been 
made, through which the diffrac- 
tion spectra pass. The result 
is, that in lieu of the ordinary 
hexagonal markings, the valve 
appears of a beautiful blue colour 
on a black ground, and covered 
with circular spots, which are 
very clearly defined with the 
deepest eye-pieces ; in fact, it is 
now so shown with a deep astronomical eye-piece made by the 
elder Dollond in the early part of this century. 
This appearance is in exact accordance with Professor Abbe’s 
theory. In answer to a letter in which I described the effect 
produced in the above experiment, Dr. Abbe informed me that 
the Philosophical Faculty of the University of Jena had pro- 
posed as a question to the mathematical students the effect 
produced in the microscope by these interference phenomena, 
and he enclosed with his reply a copy of a prize essay written 
thereon by Dr. Alfred Eichhorn. One problem was that of the 
