214 Mr Tait on the Back-Light in Portable Dioramas. 
the result of numerous and repeated elevations which have 
even continued to the historical period.* 
On admitting the Back-Light, in Portable Dioramas, upon 
different parts of a Picture at different times ; on using Light 
from Oil, §c. By GeorGE Tart, Esq., F.R.S.S.A. Com- 
municated by the Royal Scottish Society of Arts.t 
In portable dioramas, which I described in former com- 
munications to the Royal Scottish Society of Arts,{ the back- 
light was admitted behind the whole of a picture at once. I 
have now fitted up some pictures, so as to admit the back- 
light upon different parts at different times ; an arrangement 
which obviously increases materially the variety of effects 
which may be introduced into a picture, when the subject 
makes that desirable. It is done simply by means of shut- 
ters, behind different parts of the picture, attached to the 
stretching-frame, which are moved by wires or cords affixed 
to them, passing outwards through the opening by which the 
picture is inserted, and adjusted so as to be moved without 
noise. That opening may be either at the top or at the side 
of the box, as may be preferred. Tissue-paper is used along 
with the shutters, when it is necessary, in such a manner as 
to produce the effect intended. 
In dioramas constructed for internal light (as described in 
my last two communications), particularly when shutters are 
used as before suggested, it is convenient to have two counter- 
poises, just sufficient to prevent the springs from closing the 
gas stop-cocks ; which counterpoises can be attached, when 
necessary, to the cords for opening the stop-cocks, and by 
means of which the flames can be retained, during pleasure, 
* From Erichson's Archiv fiir Naturgeschichte: Zehnter Jahrgang, 
viertes Heft, p. 348. 1844. 
+ Read, and diorama exhibited, before the Royal Scottish Society of 
Arts, on 13th January 1845. 
t See vol. xxxii., p. 142; vol. xxxiii.. p. 64; vol. xxxiv., p. 275; 
vol. xxxv., p. 53; and Transactions of the Society, vol. ii., pp. 127, 162, 
215, and 230, ; 
