is got by means of a mixture of eosene, iodized collodion, and bromized
collodion. Prof. Rowland and Captain Abney, R.E., are at present engaged
in preparing a new map of the whole spectrum with a focus of 18 feet.

In reply to Mr. Hilger, F.R.A.S., the author stated that if the metal is
the true speculum metal used by Lord Rosse, it would stand the effects
of climate, he thought; but if too much copper were put in, it might
not.

In reply to Mr. Warren de la Rue, Prof. Rowland said that 42,000 was the
largest number of lines he had yet required to engrave on the metal.

Prof. Guthrie read a letter from Captain Abney, pointing out that Prof.
Rowland's plates gave clearer spectra than any others; they were free
from "ghosts," caused by periodicity in the ruling, and the speculum
metal had no particular absorption.

Prof. Dewar, F.R.S., observed that Prof. Liveing and he had been engaged
for three years past in preparing a map of the ultra-violet spectrum,
which would soon be published. He considered the concave gratings to
make a new departure in the subject, and that they would have greatly
facilitated the preparation of his map.

* * * * *




A NEW POCKET OPERA GLASS.