humanfaculty4 - page 262 of 305

enables me to regulate the burners behind it, until the image of the portrait on g is adjusted

to a standard degree of brightness.

For convenience in enlarging or reducing, I take care that the intersection of the

vertical fiducial line with that which passes through the pupils of the eyes shall correspond

to the optical axis of the camera. Then, as I enlarge or reduce, that point in the image

remains fixed. The uppermost horizontal fiducial line continues to intersect the pupils, and

the vertical one continues to divide the face symmetrically. The mouth has alone to be

watched. When the mouth is adjusted to the lower fiducial line, the scale is exact. It is a

great help having to attend to no more than one varying element. The only inconvenience

is that the image does not lie in the best position on the plate when the point between the

eyes occupies its centre. This is easily remedied by using a larger back with a suitable

inner frame. I have a more elaborate contrivance in my apparatus to produce the same

result, which I need not stop to explain.

For success and speed in making composites, the apparatus should be solidly made,

chiefly of metal, and all the adjustments ought to work smoothly and accurately. Good

composites cannot be made without very careful adjustment in scale and position. An off-

hand way of working produces nothing but failures.

I will first exhibit a very simple but instructive composite effect. I drew on a square

card a circle of about 2½ inches in diameter, and two cross lines through its centre, cutting

one another at right angles. Round each of the four points, 900 apart, where the cross cuts

the circle, I drew small circles of the size of wafers and gummed upon each a disc of

different tint. Finally I made a single black dot half-way between two of the arms of the

cross. I then made a composite of the four positions of the card, as it was placed

successively with each of its sides downwards. The result is a photograph having a

sharply-defined cross surrounded by four discs of precisely uniform tint, and between each

pair of arms of the cross there is a very faint dot. This photograph shows many things. The

fact of its being a composite is shown by the four faint dots. The equality of the successive

periods of exposure is shown by the equal tint of the four dots. The accuracy of adjustment

is shown by the sharpness of the cross being as great in the composite as in the original

card. We see the smallness of the effect produced by any trait, such as the dot, when it

appears in the same place in only one of the components: if this effect be so small in a

series of only four components, it would certainly be imperceptible in a much larger series.

Thirdly, the uniformity of resulting tint in the composite wafer is quite irrespective of the