164. | Optics. 343 
The plates are lithographed in ink of different tints, and form as 
TOE ; are: , 
perfect a representation of the spectrum lines as it seems possible to 
obtain. 
M. Van der Willigen has communicated * the results of some deter- 
minations of the indices of refraction of twelve rays of the solar 
spectrum for distilled water. Hvery precaution has been taken to 
secure accuracy, and the readings have been taken to one second, We 
give the results for the eight Fraunhofer rays :-— 
GUN ae teed 4) olan wet MB Z89O 
Bie eee Re hea 2° A I88048 
PTA Fes Pre of feet eB BRIT 
D : eet ee Bo o0 et 
ee to ah eae eee ho LOO AL 
ee eve ett ese ©. 4 le SoT 20 
Gee ee ae 0's” TE OA065 
He 5 hee OA OY a Me te SE SESEC 
A very interesting experiment in spectrum analysis has been de- 
scribed by M. Louis Grandeau.t During a stormy night he arranged 
a spectroscope at his window, so that the lightning could illuminate one- 
half of the slit, whilst one of Geissler’s nitrogen vacuum tubes was send- 
ing its light in through the other half of the slit. A small quantity of 
vapour of water which remained in the nitrogen tube at the time it was 
prepared was sufficient to produce the characteristic ray of hydrogen 
superposed on the nitrogen rays. M. Grandeau was able for an hour, at 
intervals of about five minutes, to observe the spectrum of the light- 
ning, the general appearance of which at first sight recalled that of 
the electric spark; but on closer observation he soon noticed in the 
spectrum of almost every flash the coincidence of a certain number of 
the rays of its spectrum with those of the spectra of nitrogen and 
hydrogen. This is easily understood when we remember that am- 
monia and nitric acid are produced under the influence of the electric 
discharge. 
The solar radiation has long been supposed to exercise a marked 
action upon all bodies exposed to its influence. The difference 
observed between plants which have grown exposed to its full power, 
and others which have received but a limited share of its action, is 
generally very great. M. Bourgeois has lately made some observa- 
tions on meadow grass, part of which was fully exposed to sunshine, 
whilst the other part was grown in a shady spot. After it had 
been converted into hay, that portion which had had the full benefit of 
the sun was greedily eaten by some horses, whilst they refused to 
touch that which had been grown in the shade. Upon subjecting the 
two kinds to distillation by steam the sunned portion was found to be 
much richer in odoriferous principle than the other. These experi- 
ments show that other evils besides actual paucity of crop spring from 
a dull cloudy summer and autumn, whilst sunshine, besides increasing 
* «Comptes Rendus de l’Académie des Pays Bas.’ 
t ‘Practical Instructions in Spectrum Analysis.’ Paris: Mallet Pachelier. 
2a2 
