of Edinburgh, Session 1884 - 85 . 
127 
lines in the spectrum, as the hands have not a definite boundary. 
It will he observed that neither the yellow nor the blue light 
of the spectrum passes into the box, neither of these colours being 
passed by any of the glasses. 
There is considerable confusion as to the names of the colours 
at the violet end of the spectrum, which for clearness I must refer 
to here. The colour of the violet glass in the apparatus is often 
called dark blue, while the colour that is generally known as violet 
has a reddish hue in it, and when examined with the spectroscope 
is found to have light of both ends of the spectrum. This reddish- 
violet is therefore not the violet of the spectrum, but is a compound 
colour. It is desirable that we should not give the same name to 
two different colours, and, as the spectrum is an excellent standard 
of colour, we had better call by the name of violet that colour which 
corresponds to the violet of the spectrum. It is no objection to this 
alteration to say that this colour is often called dark blue, because, 
though it is sometimes called blue, yet it is not the colour which is 
most generally recognised as blue. All blues contain green, and can 
be produced by mixing the violet of the spectrum with the green. 
The blues have a wide range, and run into violet on the one side 
and into green on the other. Prussian blue, for instance, contains 
as much green as violet ; while ultramarine blue contains compara- 
tively little green. It is, therefore, desirable that the colour, which 
has neither green nor red in it, and which contains only the light 
of the violet end of the spectrum, should be called violet. 
In experimenting with the apparatus above described, some 
practice is necessary to enable the observer to keep the eye steady 
in the correct place, as any movement tends to cause changes in the 
colour. To overcome this difficulty another form of apparatus has 
been constructed without lenses. It consists of a rather large 
camera obscura, divided vertically into two compartments by a 
thin metal partition. One compartment is lighted by an opening 
in the top, provided with a shutter to regulate the brilliancy of 
the light falling on a piece of white paper placed on the bottom 
of this compartment. The other compartment is lighted by a 
large circular window, also in the top. This window is divided 
into three sectors, and is glazed with red, green, and violet glasses. 
Each sector is provided with a shutter to regulate the amount of 
