218 Prof. Dufour : Instructions for the better observation of 
If it were possible to procure observations from the Polar 
regions during their long night of several months, it would be 
specially interesting ; and I would take the present opportunity 
of recommending this branch of study to the notice of travellers 
who may be passing one or more winters in the midst of the ice 
of a Polar region. I regret exceedingly not having begun my 
observations ten years sooner, as then those hardy mariners, who, 
in search of Sir John Franklin, spent so many winters and tra- 
velled over so great a portion of those regions, might perhaps 
have been induced to devote some of their time to this branch of 
study. But similar expeditions may be undertaken agai ; and 
if such should be the case, I promise myself to call the attention 
of future explorers to this phenomenon, in the hope of procuring 
valuable results from their observations. 
But in order to facilitate their efforts, and to avoid useless ex- 
penditure of time, as well as to render the indications given by 
one observer capable of being compared with the observations 
made by another, I would recommend the use of the following 
instructions, as the fruits of the experience acquired in the long 
series of observations taken by me at Morges. 
lst. The manner of observing. 
I have tried several scintillometers—all those indicated by 
Arago, and a new one proposed by myself; but I am convinced 
that none of them are worth the observations taken by the naked 
eye. After a little practice it becomes easy to ascertain with 
tolerable exactitude whether a star scintillates more or less than 
another star, and to indicate the amount of scintillation by a 
given number, in like manner as in meteorology the state of the 
sky, or the force of the wind, is indicated by figures. My own 
method is to designate by 0 the absence of all scintillation, and 
by 10 the highest degree, which is seen very rarely, and only 
when the star is near the horizon, when it sometimes scintillates 
very strongly, changes colour, and sometimes even disappears 
and appears again. With a little practice it soon becomes easy 
to distinguish degrees of scintillation even between O and 1, 
1 and 2, &. The next step is to ascertain the scintillation with 
still more exactitude, and to designate it as 0°4, 1:6, &c., though 
it is scarcely possible to subdivide these degrees further than 1, 
4, or 5. 
It may appear that this division is an arbitrary one, and that 
it is difficult to appreciate by figures a phenomenon like the sem- 
tillation of the stars: but this mode of proceeding is indicated 
by several learned men ; among others by the celebrated Horace 
Benedict de Saussure, who employed a similar means of estima- 
ting the famous dry fog of 1783. 
De Saussure gives the amount of intensity sometimes as 38, 
