129 
of Edinburgh, Session 1884 - 85 ". 
open the red and green shutters, the colour produced is a canary- 
yellow of considerable brightness and strength. To get this bright 
yellow we must be very careful in the selection of the glasses in the 
window, and also be very certain that every ray from the violet glass 
is excluded by its shutter, as a very little violet light causes the 
yellow to grow pale and change to white. 
An interesting and beautiful experiment can be made with this 
box. If, after having adjusted the colours to produce white, we 
introduce an opaque body into the compartment lighted with the 
coloured glasses, there is at once formed a striking display of 
prismatic-like colours. The paper on the bottom of the box is no 
longer white, but covered with most varied and brilliant hues, 
caused by the penumbra of the opaque body being lighted with 
different colours on the different sides. These effects can be 
intensified by placing over the coloured glasses a shutter with three 
round openings cut in it, so as to separate the sources of the different 
coloured lights. The colours on the bottom of the box will now be 
found to be further separated, more distinct, and fuller in tone. The 
form of the shadow-producing body may be varied, but perhaps the 
most beautiful effects are got when it is in the shape of a flat ring, 
placed a short distance from the paper. With that form if is 
interesting to follow the manner in which the different colours are 
produced. Confining our attention to the light passing through the 
centre of the ring, we can distinguish on the bottom of the box 
three overlapping circles of light — one red, another green, and the 
third violet ; where the red and green overlap a brilliant yellow is 
produced ; where the green and the violet overlap are the blues ; 
and the red and the violet give the purples; while in the centre, 
where all three circles overlap, white is produced. The purity of 
these colour effects is increased by having the inside of the box 
painted of a dull black, as this destroys all internal reflections which 
would dilute and weaken the colour effects. In order that the same 
apparatus may do for both kinds of experiments, the inside of the 
box is provided with two folding screens, each the same size as the 
side of the compartment ; one side of each screen is black and the 
other white. By folding the screens one way, the inside of the box 
is made all white, for experiments in combining colours ; and by 
folding them the other way, the interior is made all black, to prevent 
VOL. XIII. 
I 
