of Edinburgh, Session 1884-85. 
125 
In making white there is, however, always a difficulty in saying 
when it is white, as there seems to he a range of tints, all of which 
seem nearly white. To overcome this uncertainty, the instrument 
is so arranged that either of two methods may be employed to 
supply the eye with a standard of white light of the correct 
brilliancy, with which to compare the compound white. By one 
arrangement the whole field can be alternately filled with white 
light, or with the white produced by the coloured lights. By this 
means a very true white can be produced. The adjustment can be 
made so that no difference can be detected whether the illumination 
is the result of the coloured lights, or of the white light. Another 
plan is to fill only one-half the field with the coloured lights, and 
the other half with white light. 
For making these comparative tests, the opening over the 
coloured glasses is used, and a horizontal division is placed in the 
box, dividing it into two from end to end. This dividing screen 
is hinged at the front end, just above the opening filled with the 
colours. The other end of the screen next the eye-pieces is moved 
up and down by means of a lever placed outside the box; when 
the end of the screen next the eye-pieces is raised, the coloured lights 
fall on the eye-piece, and when it is lowered the coloured lights are 
cut off, and white light from the upper opening enters the lens. The 
amount of white light entering this opening requires to be carefully 
regulated, by means of the shutter, to the same brightness as the 
white produced by the colours before a match can be made. When 
properly adjusted it is very difficult to tell whether we are looking 
at natural white, or the white of the combined colours. 
When we wish to illuminate only one-half of the field with white 
light, the horizontal screen is lowered till it is half-way across the 
lens, and the division must be carried up to the stop in front of the 
lens. When this is done, the upper half of the field is illuminated 
with white light, and the lower with the combined colours, and if 
the eye is kept steady it is simply a question of very nice adjust- 
ment to make a perfect match between the two halves, and the 
field made to look like a white disc with a black bar across the 
middle. When half the field is filled with white light the con- 
ditions are very favourable for observing the various combined 
colours on the other half ; the comparison seems to make them more 
