NATURAL PHILOSOPHY. 173 



single object is quite dull and lusterless. On this experiment M. Dove founds 

 a theory of luster, supposing it to be produced by the action of light received 

 from surfaces at different distances from the eye. An ^example of this is the 

 effect observed on looking at varnished pictures : one portion of the light 

 conies from the anterior surface of the varnish, and the other from its posterior 

 surface, the action of both of these conspiring to produce the observed luster. 

 The metallic luster of mica is also referred to by M. Dove as an example of 

 the same kind. In his present communication Sir David Brewster controverts 

 the theory here laid down, and bases his objections on the following remark- 

 able experiment : where a white surface without definite boundary and a 

 black surface of the same kind are regarded through the stereoscope no luster 

 is observed. Sir David therefore infers that the luster is due not to the rays 

 from one surface passing through the other to the eye, but to the effort of the 

 eyes to combine the two stereoscopic pictures. 



STEREOSCOPIC DAGUERREOTYPES. 



TTnen two pictures, taken with two ordinary cameras, placed 2 inches 

 apart, are placed in the stereoscope, they do not exhibit that relief which is so 

 striking in pictures taken in cameras placed further apart. Now as the human 

 eyes are only separated by a distance of 2-J inches, the fact as above stated 

 has excited much attention and inquiry, ending hi controversy. Mr. Mascher, 

 a well-known photographic artist of Philadelphia, is of the opinion that an 

 explanation may be found in the fact that the lenses of the camera, are much 

 larger than the eyes, and he infers that the distance separating the camera 

 ought to be increased in porportion as their lenses are larger than the eyes. 

 In the course of his experiments, he took a camera and reduced the opening 

 by which light entered to a diameter of one eight of an inch, that being the 

 diameter of the diaphragm of the human eye. He found that the focal range of 

 the lens was thereby much increased. He then removed the lenses, and dis- 

 covered that the pictures of external objects thrown upon the ground glass of 

 the camera were very clear and distinct. Substituting a metal diaphragm 

 with an opening one fiftieth of an inch hi diameter for the picture diaphragm 

 of one eighth of an inch in diameter, he obtained an excellent picture upon a 

 prepared daguerreotype plate, without the intervention of a lens. His next 

 proceeding was to make two apertures one sixty-sixth of an inch hi diameter 

 in the same camera, and the images were thrown on the same plate. After 

 twenty minutes, exposure, good pictures were produced. These pictures pos- 

 sessed the proper amount of relief in the stereoscope, but being taken on the 

 wrong side of each other for this purpose, it was necessary to cut the plate in 

 two, and reverse then 1 relative position. 







PHOTOGRAPHIC TELESCOPE. 



In accordance with a recommendation made to the British Association, by Sir 

 John Herschel* in 1854. that daily photographic pictures be taken of the sun's 

 disk, for the purpose of studying by comparison its physical features and changes, 



* See Annual of Scientific Discovery for 1855, p. 203. 



