_Photographic Exhibition._--We understand that the Photographic Society has
made arrangements for an exhibition of photographs in the metropolis during
the months of January and February next. The exhibition will not be
confined to the works of native photographers, but will comprise specimens
of the most eminent foreign artists, who have been specially invited to
contribute. From the advances which have been made in this favourite art,
even since the recent exhibition in the rooms of the Society of Arts, we
may confidently anticipate that the display on the present occasion will be
one of the highest interest.
_How much Light is obstructed by a Lens?_--Can any of your scientific
correspondents furnish me with an approximation to the quantity of light
which is transmitted through an ordinary double achromatic lens, say of
Ross, Voightlander, or any other celebrated maker?