of Edinburgh, Session 1884-85. 
123 
now in use. The colours produced by them are very brilliant and 
beautiful, and the secondary ones, such as yellow, blue, purple, are 
brilliant and at once recognised by the eye, not being of that dull 
low tone given by the rotation of coloured discs, while the white is 
so pure and bright that it cannot be distinguished from that produced 
under ordinary conditions. 
In two forms of this instrument a lens or lenses are used for 
combining the colours. One of these instruments is simply a 
camera obscura with a single lens. Suppose now there is a bright 
white object in front of the lens. The rays coming from any point 
of that object pass through all parts of the lens, some through the 
upper half and some through the lower, after which they are again 
brought to a focus on the field of the camera. If, now, we place in 
front of the upper half of the lens a transparent red screen, then 
half of the entering rays will be red and the other half uncoloured, 
and the image will be of a reddish hue. Let us now place in front 
of the lower half of the lens another screen of a different colour- 
say green; the one-half of the entering rays will now be red, and the 
other half green, and these being combined on the field we get the 
resultant colour, namely yellow. In this way we can combine on 
the field of the camera any colours we desire, and by altering the 
amount of the lens covered by the different screens we can combine 
the colours in any desired proportion. 
Another method of combining colours by means of lenses is to 
look at the colours through a short focused lens, the coloured 
objects being placed at a considerable distance. Suppose we have 
a long camera obscura, closed at one end, and having a short 
focused lens fitted into the other end. If we now make a small 
opening anywhere near the axis of the lens in the far end of the 
camera, so as to admit a little light, and look at the opening through 
the lens, we shall see a round disc of light, but the disc is not seen 
exactly in the direction of the opening, but in the axis or centre of 
the lens. Now make another small opening also near the axis of 
the lens, say on the side opposite the first. If we now look through 
the lens, we shall not see two discs of light, but apparently only 
the one we saw at first, but now it is brighter. We may go on in 
this way making a number of holes, and the light passing through 
them will be combined into one round disc of light. Now place 
