Physiology. — “On the Observation and: Representation of Thin 
Threads’. By Prof. W. EINTHOVEN. 
(Communicated at the meeting of September 25, 1920). 
A fall discussion of this subject will be published elsewhere; a 
few conclusions, however, may be given here. 
|. Threads of 0.1 to 0.2 u can easily be observed with the naked 
eye as light lines on a dark background. Without difficulty they 
can be shot or blown, fixed, transferred, put under the microscope, 
bombarded, and stretched out in the galvanometer. 
2. Any thread that can exist, however thin it may be, can be 
made ultra-microscopically visible, when we are only able to bring 
it under the microscope in an efficient way. When it is assumed 
that in case of uniform radiation of a thread the quantity of light 
reflected by it, decreases in direct ratio to its diameter, the dia- 
meter of the thinnest thread visible is calculated at 0,2 « 10-6 uu, 
By way of comparison it may be said that the diameter of a hydrogen 
molecule is about a million times larger. 
3. The power to see the thinnest dark thread against a light 
background with the unaided eye is not determined by the dimen- 
sions of the cones on the retina, but by the power to distinguish 
two degrees of brightness. Two luminous points or luminous lines 
which approach each other more and more are still observed sepa- 
rately when they are represented on the retina at a distance apart 
corresponding to the diameter of a cone at which they appear at a 
visual angle of 60”; a thread, however, can still be seen at an 
angle of 2". 
4. Every circumstance which renders the microscopic image of a 
dark thread against a light background less sharp, increases the 
apparent diameter of the thread. As no microscope comes up to 
ideal demands, it may, therefore, be assumed, that the results of 
the measurements made with this instrument either agree with 
reality or give too high values, so that the threads mentioned in 
this paper are really 0,1 or 0,2 w thick or thinner. 
5. The conditions to observe the thinnest dark thread against a 
light background with the microscope, and represent it, are different 
from those which hold for seeing two luminous points or lines separate 
