318 Intelligence and Miscellaneous Articles. 
cent, of short-wave radiation reaches us under similar circumstances. 
The intermediate rays are transmitted in amounts proportional to 
some direct function of the wave-length. It seems almost certain, 
from Tyndall's experiments above quoted, that these short waves 
are not absorbed in the ordinary acceptation of the term. What 
then becomes of them ? 
In 1869 Tyndall demonstrated that if ordinary light fall upon a 
cloud of suspended matter in sufficiently great subdivision, the long 
waves pass through unhindered, whde the blue are reflected ; and 
from this he deduced an explanation of the colour of the sky. In 
1880 I published a brief paper experimentally extending these 
results to ordinary coatings of oxides of the metals projected upon 
charcoal by a blowpipe-flame. From these it was shown that an 
oxide in thin layers might reflect only a beautiful blue ; that if the 
coating naturally absorbed blue, it reflected in preponderance in 
thin layers the next longer wave-length ; and that, in fact, the 
amount and character of reflection depended upon the size of 
particles and thickness of layer. No quantitative measurements 
were made ; but from the magnificent character of the blues obtained, 
I concluded that nearly all the incident light of that wave-length 
was reflected. Applying this theory to the sky, we have an expla- 
nation not only of its blue colour and of its sunset tints, but also 
of Prof. Langley's results that through the lower air, full of these 
floating particles, the rays of shorter wave-length do not penetrate. 
It is not selective absorption ; it is selective reflection. — Johns 
Hopkins University Circular, August 1883. 
OX THE RECIPROCAL EXCITATION OF ELASTIC BODIES TUNED TO 
NEARLY THE SAME PITCH. BY DR. G. KREBS. 
When two strings tuned perfectly alike are stretched upon the 
same monochord, it is well known that each is capable of causing 
the other to vibrate with it. The same is true of two tuning-forks 
of exactly the same pitch placed opposite each other on two reso- 
nators. 
But if the pitch of two strings is not exactly the same, the 
deeper-toned one can excite the higher, but not the higher-toned 
one the lower, provided that the difference between the numbers of 
vibrations amounts to at least 2 or 3, at most 3 or 4. 
Conviction of this is obtained when the strings are first brought to 
exactly the same pitch, and then the tension of one of them some- 
what relaxed : if now one of the strings be struck or pulled in the 
middle with the finger, after a paper rider has been placed on the 
other, it can be distinctly seen that the rider leaps from the higher- 
toned string if the difference of the numbers of vibrations amounts 
to 3 or 4, while, conversely, the rider upon the deeper-toned string 
remains almost unmoved when the higher-toned is struck. 
If the difference'of the numbers of vibrations amounts not to 2, 
each string is in a condition to excite the other ; but the deeper- 
