1904 - 5 .] Dr J. Halm on Astronomical Seeing. 459 
I should like, therefore, to discuss it here in a few words, especially 
as the answer to it seems to he simple and conclusive. 
Let us first get an insight into the cause of the blurrings 
of telescopic images, so far as atmospheric circumstances are 
responsible for it. We feel no hesitation to look for this cause 
in the incessant motions of our atmosphere, in the spontaneous, 
fitful, and ever varying displacements of air from one place to 
another, in consequence of local changes of temperature and 
pressure. Now, the motion itself can have no direct effect on the 
definition. The cause of the blurring must be looked for in sudden 
changes of the index of refraction of the air resulting from its 
internal motions. If, for instance, a volume of heated air rises 
from the surface of the soil to a higher layer, and arrives there 
with a temperature higher or lower than that of the layer itself, 
the temperature and density of that particular point of the 
atmosphere, and thus its index of refraction, will be momentarily 
altered. Hence the direction of a ray of light passing through this 
point must suffer a corresponding change ; the consequence being, 
that among the rays which, under undisturbed and perfectly ideal 
conditions, would all reach the object-glass in parallel directions, 
those passing through the affected area will be thrown into slightly 
different paths, and will therefore be focussed at different points of 
the field of view. 
Now, we may ask : If the definition of telescopic images 
depends on these fitful changes of the index of refraction which 
are caused by the unavoidable movements and displacements of 
air in the atmosphere, are there conditions under which these 
movements have a minimum disturbing effect ? It is well known 
that there is indeed one particular state of the atmosphere in 
which these conditions seem to be present, viz., the so-called state 
of adiabatic equilibrium. In this state a volume of air carried 
from one layer to another will arrive at its new position with 
exactly the same temperature and density which were previously 
possessed by the mass of air whose place it has taken. Hence 
motion of air, in whatever direction it may take place, is not 
accompanied by change of the index of refraction. We may 
compare the atmosphere in this particular state to a liquid in 
which bodies are suspended, of any size and shape, but of the 
