282 APPENDIX. [No. xix. 



the fact, that even the apparent imperfections in the construction of the 

 parts of our body are necessary for the highest integrity of their functions. 



" It occurred to me that, if the laws of binocular perspective were 

 correct, pictures might be obtained by photography which should repre- 

 sent the appearances observed with both eyes. The conditions required 

 for binocular sun-pictures are similar to those required for binocular 

 drawings or paintings. As it is requisite that there should be one point of 

 sight for the two perspective drawings, considerable nicety is required in 

 the production of binocular photographs, as the slightest deviation from 

 correctness produces doubleness or great distortion. To obtain a binocu- 

 lar picture of any body, the camera must be employed to take half the 

 impression, and then it must be moved in the arc of a circle of which the 

 distance from the camera to the point of sight is the radius, for about 

 2-2 inches, when a second picture is taken, and the two impressions, con- 

 jointly, form one binocular picture. 



" There are many ways by which this result may be obtained. A spot 

 may be placed in the ground glass, on which the point of sight should be 

 made exactly to fall; the camera may then be moved 2 2 inches and 

 adjusted till the point of sight falls again upon the same spot on the 

 ground glass, when, if the camera has been moved in a true horizontal 

 plane, the effect of the double picture will be perfect. 



" For obtaining this motion in the true arc of a circle, Mr. Hensman, 

 the engineer to the Bank of England, recommended me to adopt a car- 

 riage with two movable axles, with wheels of which the front pair is a 

 little smaller than the back pair. The idea of the construction is, that 

 the carriage should revolve on two cones which run round a circle, and the 

 diameter of the circle is determined by the distance between the larger 

 and smaller wheels and the convergence of the axles. In practice, on a 

 surface adjusted by spirit-levels, it answers well, and probably may be 

 found useful in some cases. At Messrs. Home and Thornthwaite's photo- 

 graphic room, an apparatus has been fixed, which allows the motion of the 

 camera to be made perfectly horizontal in the arc of a definite circle. 

 From experiments which we have made, I rather give the preference to 

 pictures made with the camera in continual motion,* backwards and for- 

 wards, for 2 2 inches, as the picture is, in this case, even more beautiful 

 than if the two images were superimposed. This experiment is very 

 remarkable, for who would have thought formerly, that a picture could 

 possibly have been made with a camera in continual motion ? Neverthe- 

 less, we accomplish it every day with ease, and the character of the like- 

 ness is wonderfully improved by it. 



" Whenever a solid body is depicted in binocular perspective, a suitable 

 background should be arranged behind it to exhibit the interpenetration. 

 If this be not considered, the picture has an increased width with double 

 edges, and does not exhibit that glorious delicacy of shading which Nature 

 gives to objects seen with both eyes. In all cases of binocular perspective 

 we must be careful not to imitate Nature by endeavouring to depict an 

 excessive range of distances. In practice, the eye can only focus objects 

 within a certain range : hence in pictures we still take Nature as our guide 

 when we only depict a moderate rouge. 



"It is not easy to predict the extent or the importance of this mode 

 of drawing, because an extensive experience is required before artists can 



