AMERICAN INSTITUTE. 655 



As an illustration (for there are some here who, probably have 

 not paid much attention to the science of optics), I will suppose 

 that a room be dark, and a small opening in the shutter through 

 which a very fine pencil of light enters; at a distance from this is 

 placed a white screen, which receives the light and exhibits a 

 bright spot, blit upon close examination, it will be observed that 

 the spot is not like a piece of white paper cut out and fixed upon 

 a black ground, but exhibits an indistinct outline, with colored 

 fringes on each side; and should a wire or thread be now drawn 

 through this beam of light, close to the opening, the shadow from 

 it will be far from sharp, but will exhibit a blurred image colored 

 on each side by fringes in the same manner, and these mixing 

 with the fringes of the circle, give rise to that indistinctness which 

 may be seen on all images thrown on a screen by the solar mi- 

 croscope. Every device that mathematics could suggest in the 

 configuration of the lenses, have proved ineffectual in correcting 

 this species of imperfection. But to return to the apparatus of 

 Mr. Moore, in which this difficulty is not encountered, and which 

 I will now describe. The light is not passed through the nega- 

 tive, and consequently near to innumerable opaque bodies, but is 

 reflected from the surface, thereby avoiding any interference with 

 the rays in their passage to the tablet or canvas. This apparatus 

 is so arranged that the sun-light falling on a mirror, is reflected, 

 and condensed, upon a small daguerreotype or other picture, by 

 which means it is strongly illustrated; directly in front of this is 

 fixed a common small size camera tube, so situated that its axis 

 is at right angles to the plane of the picture, and being adjustable, 

 a very sharp image is thrown upon the tablet, free from colored 

 fringes and overlappings. 



The difference between the two methods will at once be seen 

 to consist in the fact that Mr. Moore receives upon his canvas a 

 reflected image, retaining all the perfection and sharpness of the 

 original, while by the method now used, a transmitted image is 

 received, with all its attendant imperfections. As a familiar illus- 

 tration, it is well known to the practical photographic printer, 

 that should the glass negative be placed, in the printer's frame 

 with the collodion up, and the paper placed upon the opposite 



