Photographic Researches and Portraiture 289
for all subjects, but the distance from interpupillary line to the line of the lips is also made constant'. The general principle of the apparatus is that of a modified copying-camera, only the alteration in scale and the adjustment in position are not done on the focusing-screen, because it is desirable that the negative which is to be several times exposed should remain fixed in position. By the simple artifice of a mirror let down at 45° across the camera, Galton gets an image on a horizontal screen in the roof of his camera, and upon this screen also are thrown the three fiduciary lines which serve as register-marks for his adjustments. The details of the apparatus will be sufficiently indicated by the accompanying line engraving and Galton's description of it ; we may merely remark that gas would now-a-days be probably replaced by electric light. The apparatus is still preserved in the Galton Laboratory.
DIAGRAM SHOWING THE ESSENTIAL PARTS. A A
The body of the camera, which is fixed. Lens on a carriage, which can be moved to and fro.
Frame for the transparency, on a carriage that also supports the lantern ; the whole can be moved to and fro.
r The reflector inside the camera.
rn The arm outside the camera attached to the axis of the reflector; by moving it, the reflector can be moved up or down. A ground-glass screen on the roof, which receives the image when the reflector is turned down, as in the diagram.
e The eye-hole through which the image is viewed on g; a thin piece of glass immediately below e reflects the illuminated fiducial lines in the transparency at f, and gives them the appearance of lying upon g-the distances fk and gk being made equal, the angle fkg being made a right angle, and the plane of the thin piece of glass being made to bisect fkg. Framework, adjustable, holding the transparency with the fiducial lines on it.
t Framework, adjustable, holding the trans
parency of the portrait.
"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 offhand way of working produced nothing but failures." (p. 143.)
Galton exhibited certain results of very considerable interest tending to meet criticisms which had been raised. He drew on a square card a circle of about 2.5 inches diameter with a vertical and a horizontal diameter. Where these diameters met the circle he placed four circular discs of different tints, and in one quadrant he placed a black dot. He then made a composite
' It would probably be possible by a slight rotation of the frame for the transparency about a central vertical axis to make the interpupillary distance (or the. external ocular distance) constant without in any way injuring the result.