1903-4. ] ULTRAMICROSCOPIC ORGANISMS. 61 
neglected, is 0.05 so that our organism of contagious pleuro-pneumonia 
must be in size somewhere between 0.05 micron and 0.25 micron ; all the 
others must be under 0.05 micron. With ordinary methods they must 
always remain invisible, but about a year ago Siedentopf and Zsigmondy 
with the assistance of the Zeiss firm constructed an optical device which 
may lead us some distance in determining the approximate size of these 
parasites although it will never reveal to us anything of their structure. 
These investigators wished to study the condition of colloidal gold in the 
so-called gold ruby glasses ; the point to be determined being whether 
the gold was distributed as discrete particles or continuously through the 
glass. They attacked the question in the same manner as Tyndall many 
years ago attacked the question of the presence of fine dust particles in the 
atmosphere, viz., by the use of a fine-pencil of sunlight. 
By means of a heliostat, a spectroscopic slit aperture and a series 
of condensing lenses, a very fine slit of light was thrown into the glass at 
right angles to the line of vision. The plane of light was observed by means 
of a very high power microscope and when this was sharply focussed the 
light plane appeared as a dark ground filled with enormous numbers of 
brilliant particles. Eech particle showed no structure, simply a diffraction 
disc due to the light being turned and thrown into the tube of the micro- 
scope, but nevertheless counts could be made and the size of the particles 
could be estimated. 
The unit of measurement which they take is that used by the phy- 
sicist and chemist, viz., the millimicron, written my zyoobooo mm, or 
tooo Micron, and they found the size of the particles in the following 
manner. It can be demonstrated that the diffraction disc from particles 
smaller than .006 microns (iooé000 mm) would be too small to see, conse- 
quently the size must vary between .25 micron and .o06 micron. The 
number in a given cubic area of glass was counted, the amount of gold 
estimated chemically, and from these data and the specific gravity of gold 
the probable size calculated. By this means the smallest particles seen 
were estimated to be about .006 micron, whilst the average size was about 
.05 micron. 
Now if we remember for a moment the estimated size of a molecule, 
varying from say o.5 millimicron for the largest, or according to other 
authors 2 millimicrons for a molecule of albumen, down to .05 millimicron, 
we see that the smallest particles that this method can reveal are still some 
distance from molecular size. 
But if the method were applied to our ultramicroscopic particles it 
might be possible to estimate how closely they approach the size of a 
molecule. 
