July 30, 1886.] 



SCIE^'CE. 



91 



colored glasses (by far the fairest test), confirm 

 those of Galton. 



Experiments of this kind are far more satisfac- 

 tory than those in which composite portraits are 

 made from the same components taken in differ- 

 ent orders : for one has to decide in the one case 

 merely on the identity or difference of tint of 

 disks or rectangles placed side by side on the same 

 plate ; in the other, of faces with tlieir manifold 

 detail. 



Answers to both of these questions as to order 

 and time of exposure would be found in knowl- 

 edge of the rate at which light acts upon the sil- 

 ver salts of the photographic plate. 



If the rate of this action is constant up to the 

 point of a ' full-timed ' plate, then the order in 

 which the negatives are taken can make no differ- 

 ence, provided each successive fractional exposure 

 is of equal length, and the image is in each case 

 equally illuminated. If the velocity with which 

 the chemical action proceeds is not constant, then 

 the order will obviously make a difference in the 

 result, unless the exposiu-es are prolonged or 

 shortened, or the illumination made stronger or 

 weaker, as the velocity decreases or increases. 



As far as I am aware, we have no knowledge of 

 the rate of chemical action in this instance, except 

 that which is given by the experiments above re- 

 ferred to, and which points to a constant rate of 

 action within the Umits of ordinary photographic 

 exposures. Thus Galton's process appears as a 

 valuable auxiliary in the investigation of an inter- 

 esting point of the obscure field of photographic 

 chemistry. 



The possibility of the ' preiDotency ' of some indi- 

 vidual of the group as a disturbing element was 

 suggested in -Science, v. No. 118, and has since been 

 discussed by Mr. Jastrow in vol. vi. No. 134. Since 

 the composite portrait is the result of the action of 

 light on the silver salts, it would seem plain 

 that no one face, however ' individual,' ' powerful,' 

 or ' characteristic ' it may be, can be prepotent in 

 controlling the result. We must conclude that 

 the apparently prepotent face is merely a close ap- 

 proximation to the type or average of the group. 



In the hope that more may be induced to do 

 something in composite photography, I would say 

 that excellent results can be obtained with an ap- 

 paratus which is by no rueans elaborate or costly. 

 A camera for the purpose can be made of soft 

 wood by any skilful carj^enter. It need differ 

 from the usual form only in having a mirror 

 which is hung within so that it can swing down 

 to an angle of 45° for the adjustment, and up 

 against the top for exposures ; and an opening in 

 the top, over which a ground-glass plate is fixed. 

 On this ground glass the fiducial lines are drawn 



in lead-pencil, and the images focused and ad- 

 justed. It must be at the same optical distance 

 from the lens (the light being reflected to it by the 

 mirror) as the ground glass at the back of the 

 camera. A piece of ground glass placed behind 

 the negatives will serve very well in place of a 

 condensing lens for lighting them, and it is not 

 necessary to enclose the gas jet in a lantern. 



In order to give accurately timed exposiu-es, I 

 use a pendulum consisting of a wooden I'od with 

 sliding weights above and below the jaoint of sus- 

 pension, and having an arm at right angles to it. 

 At the extremity of this arm is a screen of card 

 or ferrotype plate, which, when the pendulum is 

 swinging, plays ujj and down in front of the 

 camera tube. Matters are so arranged, that, when 

 the pendulum is at rest, the lower edge of the 

 little screen lies across the horizontal diameter of 

 the tube. After the negative is adjusted, the 

 screen is held down so as to cover the end of the 

 tube, while the slide in front of the sensitive plate 

 is drawn, and then released and allowed to make 

 a double vibration. The time of exposure is that 

 of a single vibration of the pendulum, and this 

 is regulated by adjustment of the sliding weights. 



I find, as others have doubtless found, that the 

 best composites are obtained from very ' dense ' 

 negatives. Those from which the composites in 

 this number were taken were made for me by Mr. 

 Lovell of Northampton, who succeeded admirably 

 in obtaining strong negatives of very uniform den- 

 sity. John T. Stoddaed. 



NATIONAL EDUCATION ASSOCIATION. 



In point of numbers, the National education 

 association meeting at Topeka, Kan., was among 

 the most important ever held. As far as perma- 

 nent educational literature is concerned, however, 

 the contributions hardly correspond to the size 

 of the gathering. The real value of such meet- 

 ings must always be found in the quiet friction of 

 mind with ruind, and in the informal talks where 

 men learn the experience of their fellow-teachers 

 and become acquainted with the educational 

 sentiment of distant sections. There is no better 

 place than these to feel the educational pulse, and 

 learn the temper of teachers on mooted points. 



Both in the association and the council that 

 preceded, the subject of industrial education was 

 discussed at great length and with the widest 

 divergence of opinion. Dr. S. H. Peabody of 

 Illinois presented the report, which was an ad- 

 mirable paper, clearly and without i)rejudice out- 

 lining the theory of industrial education. To an 

 outsider this whole question seems unnecessarily 

 forced to the front. Only three per cent of our 



