COM P SITE P OR TRAITS. 



461 



stereoscope, and that a person who is used to handle instruments will find a com- 

 mon double eye-glass fitted with stereoscopic lenses to be almost as effectual and 

 far handier than the boxes sold in shops." 



Mr. Spencer, as he informed me, had actually devised an instrument, 

 many years ago, for tracing mechanically longitudinal, transverse, and 

 horizontal sections of heads on transparent paper, intending to superim- 

 pose them and to obtain an average result by transmitted light. 



Since my address was published, I have caused trials to be made, 

 and have found as a matter of fact that the photographic process of 

 which I there spoke enables us to obtain with mechanical precision 

 a generalized picture ; one that represents no man in particular, but 

 portrays an imaginary figure, possessing the average features of any 

 given group of men. These ideal faces have a surprising air of reality. 

 Nobody who glanced at one of them for the first time would doubt its 

 being the likeness of a living person. Yet, as I have said, it is no such 

 thing ; it is the portrait of a type, and not of an individual. 



I begin by collecting photographs of the persons with whom I pro- 

 pose to deal. They must be similar in attitude and size, but no exact- 

 ness is necessary in either of these respects. Then by a simple contriv- 

 ance I make two pinholes in each of them, to enable me to hang them 

 up one in front of the other, like a pack of cards, upon the same pair of 

 pins, in such a way that the eyes of all the portraits shall be as nearly 

 as possible superimposed ; in which case the remainder of the features 

 will also be superimposed nearly enough. These pinholes correspond 

 to what are technically known to printers as " register-marks." They 

 are easily made ; a slip of brass or card has an aperture cut out of its 

 middle, and threads are stretched from opposide sides, making a cross. 



1I 



Fig. 1. 



Two small holes are drilled in the plate, one on either side of the 

 aperture. The slip of brass is laid on the portrait with the aperture 

 over its face. It is turned about until one of the cross-threads cuts the 

 pupils of both the eyes, and it is further adjusted until the other thread 

 divides the interval between the pupils in two equal parts. Then it is 

 held firmly, and a prick is made through each of the holes. The por- 

 traits being thus arranged, a photographic camera is directed upon 



