280 APPENDIX. [No. XIX. 



tolomeo's, "hairy edge;" in Caravaggio's, "shade over line, colour;" in 

 Perugino's, " hard outline ; " in Guercino's, " indeterminate outline," &c. 



My father was greatly interested in this celebrated collection of 

 pictures in the Vatican, and spent considerable time over them. Unfor- 

 tunately the above notes, with the sketches attached to them, were con- 

 signed for care in the writing-case of my travelling bag, and have 

 remained there forgotten for several years. Lately, in thinking over 

 events in my dear father's life, the remembrance of his visit to the 

 Vatican and of these notes suddenly flashed across my mind. Had he 

 lived, he would probably have again brought forward his theories on 

 Binocular Perspective, with these notes fully set forth as illustrations. 

 He gives eleven rules or laws on Binocular Perspective : 

 " 1. The point of sight appears the same to both eyes as to one. 

 " 2. Small objects of less width than the distance between the pupils 

 of the eyes, when placed in a plane before the point of sight, are increased 

 in width and rendered either wholly or in part transparent, according to 

 their distance from the eyes. 



" 3. Large objects, in a plane before the point of sight, are increased 

 in width, and their lateral edges become transparent and allow objects to 

 be seen through them. 



" 4. Objects or parts of objects on either side the point of sight are 

 increased in width, and the edges become transparent. 



" 5. Objects in a plane, behind the point of sight, are seen in two places, 

 but indistinctly, because they are out of focus, and because their images 

 fall upon the internal surface of the retina at a greater or less distance 

 from the point of distinct vision. 



"6. Solid bodies or parts of solid bodies, appearing transparent, 

 modify the tints of objects seen through them. 



" 7. Bodies of a light colour throw a light veil over objects seen behind 

 them. Bodies of a darker colour throw a dark veil. 



" 8. Colours of different character, as yellow and blue, when super- 

 imposed according to the preceding law, produce a tint different from 

 either, and yet not the colour which would arise from their admixture. 



" 9. In cases where objects or parts of objects are widened and 

 rendered transparent, the breadth of the distinct 

 or solid part is narrower than when viewed by 

 one eye alone. 



" 10. Small objects placed some distance 

 before the point of sight and near the eyes 

 appear in two places, but one impression is 

 generally neglected. 



"11. Parts of objects becoming transparent 

 have frequently much light reflected from them, 

 and where the image is seen in two places some- 

 times the light is only reflected from one image. 



" The annexed figure will serve to give an 

 illustration of the interpenetration of objects 

 when seen by two eyes, and will also explain the 

 geometrical law on which it is founded. It will be 

 seen that by two eyes we are enabled to see a 

 greater amount of the back object than would be discernible by one eye 



