SCIENTIFIC SUMMAEY. 
329 
had very nearly, if not quite, discovered the achromatic defects of* the eye. 
Dr. Young ascribes to Wollaston the merit of having observed that when a 
luminous point is viewed through a prism, the blue end appears to be wider 
than the red, the eye being incapable of recognising that the spectrum has 
the same width throughout its entire length. An experiment was exhibited 
to show the relative distinctness of a dark line on grounds of various colours. 
A string or wire was so arranged that its shadow traversed the entire length 
of the spectrum, which was thrown on a screen by an electric lamp. When 
viewed from a short distance the edges of the shadow appeared to be sharp 
at the red end, but gradually became less distinct, until at the blue end 
nothing but a blurred line remained. 
The Velocity of Light . — Professor Cornu gave a recent lecture on this 
subject before the Poyal Institution (May 7, 1875). In this he described 
his recent experiments on the determination of the velocity of light. He 
gave an account of the method of Foucault, and exhibited the complete 
apparatus, including the arrangement of mirrors for multiplying the distance 
through which the luminous ray passed between the two reflections from the 
revolving mirror. He described the toothed wheel of Fizeau, and the im- 
provements which he had himself made in his own determinations by this 
method. He found that it was impossible to give a uniform motion to the 
toothed wheel, and therefore adopted an electrical registering apparatus to 
mark the increase of its velocity, an electric signal enabling the observer to 
point out the instant at which the right velocity is obtained. Another very 
important improvement is the substitution of a pair of observations of the 
return rays for the single observation of a total extinction. Prof. Cornu’s 
most recent determination was made in the summer of 1874, the two 
stations being the Paris Observatory and the Tower of Monthery, 14^ miles 
apart. A mean of 508 experiments gave 300,400 kilometres, or 186,660 
miles per second. 
The Aurora! s Spectrum. — Mr. J. P. Capron has (“ Philosophical Magazine,” 
April, 1875) described the results of comparison of auroral spectra with 
the spectra of hydrogen, carbon, oxygen, air, phosphoretted hydrogen, and 
iron. Mr. Capron considers that the conclusion of Angstrom, that the 
“ moisture in the region of the aurora must be regarded as nil ” cannot be 
maintained. He sums up the present state of our knowledge of the aurora 
question as follows : — u The yellow-green line, and possibly also the red, are 
due to phosphorescence or fluorescence ; the fainter lines are partly due to 
the air spectrum, and the remaining bands or lines may be due to phos- 
phorus and iron, the close coincidences in this latter spectrum with the 
lines being very striking.” 
Physical Science Popularised . — The “Chemical News” (May 28, 1875), 
quoting from “ Iron,” very justly calls attention to a scandalous ignorance 
of science lately exhibited. It says : u It is one of the most hopeful signs 
of the times that everybody is now supposed to know a little science. 
Some of us know very little. Others know a good deal, but the arrange- 
ment is somewhat confused. We scarcely know to which class the com- 
piler of the 1 Yorkshire Exhibition Guide ’ belongs. Whatever amount of 
scientific knowledge he possesses, he certainly has the art of ‘ combining 
his information,’ and presenting it to his readers in a fresh, cheerful, and in- 
