Mr Tait on the Back-Light in Portable Dioramas. 215 
at any height desired; and the hands may thus be left at 
liberty to move any other part of the apparatus. 
I may take this opportunity of mentioning, that, instead 
of using the front slider of tissue-paper, immediately behind 
the pictures, formerly suggested (at N O of the diagram”*), it 
is more simple and convenient to attach tissue-paper to the 
back of the stretching-frame of any picture for which that is 
necessary, in order to produce uniformity of effect by the 
back-light, on account of any object, for example the moon, 
being made to transmit the light without diffusing it. It 
seems better not to use tissue-paper behind a picture, unless 
it be necessary, as it intercepts more than a third part of the 
light. 
If oil, instead of gas, be used for lighting a diorama inter- 
nally, there are practical objections, unnecessary to be here 
detailed, to lighting the pictures by the direct rays from the 
flames. But they may be lighted by means of any substance 
which transmits light abundantly, and diffuses it sufficiently 
(for example, one or two plies of glass, coarsely ground on 
both sides), applied to an opening in a screen in front of 
each of the flames, in a line between the flame and the pic- 
tures, placed very near the flame, and having the light con- 
centrated upon it by a reflector behind, in a continuation of 
the same line. The quantity of light admitted upon* that 
substance is modified to any extent by a slider or sliders, 
properly formed and adjusted, on the side of the opening 
next to the flame. Those two surfaces, thus enlightened and 
thus darkened, are the sources of the front and the back 
light to the pictures, and occupy the places of the gas flames. 
But gas is, in all respects, so very much preferable to oil for 
lighting a diorama internally, and is now in so general use, 
that it seems unnecessary to enter more into detail with 
regard to the application of oil to that purpose. 
EDINBURGH, December 26, 1844. 
* See vol, xxxiv., p. 276. 
