434 
POPULAR SCIENCE REVIEW. 
atmosphere is highly charged with moisture, the aqueous vapour gives 
rise to a hand of dark lines near the red end of the spectrum. A group of 
lines is very conspicuous, and may be termed the rain-band. This rain- 
hand increases in strength in passing from colder to warmer climates. The 
information thus obtained spectroscopically is, in some sort, equivalent to 
that derived from the readings of the wet-and-dry bulb thermometers, and 
the subsequent calculations necessary to determine the hygrometric state of 
the atmosphere. — Astronom. Reg., Sept. 1877. 
Chromatic Aberration of the Rye, and Perception of Distance — No one now- 
a-days supposes that the human eye is a perfect optical instrument, at least 
in the sense in which an optician speaks of an instrument as perfect. The 
eye possesses, for example, decided chromatic aberration. Rays of light of 
dissimilar colour are not brought at once to an exact focus upon the retina, 
but each colour has its own focal distance. Let a red object and a blue 
object, of equal size, stand side by side, and it will be found that the images 
of these two cannot be in focus at the same time ; therefore the two objects 
appear to be unequally distant, or of unequal magnitude. Hence estimates 
of distance founded on apparent magnitude are liable to be rendered falla- 
cious by the colour of distant objects. And, on the other hand, estimates of 
distance founded on colour are liable to be confused by apparent magnitude. 
So far as our ideas of distance are dependent on the accurate focussing of 
rays upon the retina, it is evident that a source of error must be thus im- 
ported into the data of our perceptions. The subject has lately been very 
ingeniously handled by Mr. S. P. Thompson, of the University College of 
Bristol. He enumerates the various data for forming an estimate of distance 
which are dependent upon the eye, and not upon the limbs. After discussing 
the respective values of these data under various circumstances, he inquires 
how far they may be dependent upon the colour of an object, or upon the 
formation of an exact focus on the retina. Mr. Thompson concludes that the 
muscular sensation of adjustment of the eye to the focus of its lenses affords a 
possible means of estimating distances. When binocular methods, and those 
dependent on association of visible form and magnitude, fail, then the eye 
falls back upon colour as a means of effecting this. In fact, colour may in 
some cases outweigh the evidence of binocular vision. The chromatic aber- 
ration of the eye accounts for the well-known opinion of artists that blue is 
a retreating colour, and red an advancing colour. Aerial perspective is in- 
deed a true expression of a physical fact in the perception of distance. Mr. 
Thompson’s paper is interesting as offering a scientific explanation of certain 
empirical rules of artistic practice, relative to the expression of distance in 
painting. — Philosophical Magazine, July, 1877. 
Diamagnetism of Hydrogenium. — When the late Professor Graham suc- 
ceeded in charging palladium with hydrogen, or in u occluding ” the gas so as 
to form what he regarded as an alloy of palladium and hydrogenium, he 
was puzzled with the anomalous magnetic behaviour of the new alloy. It 
is well known that palladium, like iron, is a magnetic metal ; but it is 
believed that hydrogen is diamagnetic — that is to say, it is repelled instead 
of being attracted by the poles of a magnet. Hence it might be assumed, 
a priori, that the diamagnetic hydrogen would tend to neutralize the mag- 
netism of the palladium, and that the alloy would consequently be less 
