180 Mr Davidson on (he Process of Daguerreotype, 



lens must be shorter in the centre than at the extremities, 

 which I shall endeavour to explain by the following diagram. 

 (See Plate III. fig. 1.) 



Let AB C D (Fig. 1) be a Daguerreotype camera -box, E F the 

 lens or achromatic object-glass, HI an object placed before the 

 camera, and DD the picture formed in the focus of the lens. 

 Now, it must be evident to all, that, in order to give a flat pic- 

 ture, the focus of the lens must be longer at the edges than 

 at the centre, in proportion as the edge of the picture at B 

 and D is greater in distance from the lens than the centre at 

 g. The lens must therefore be so constructed as to have a 

 focus gradually increasing in length from the centre towards 

 the circumference. Be the lens, however, as perfect as it may, 

 yet, by attending to Fig. 1, we shall find that it will not even 

 then give a perfect focal image. For, suppose a ray of light 

 from the object I to fall on the centre of the lens at c, this 

 ray, after refraction, cannot reach the picture at B, but its 

 focus will be at a, seeing the centre of the lens is shorter in 

 focus than the edge. Let another ray of light from the object 

 at I fall on the extremity of the lens at E, we shall find its 

 focus at b. Thus the difference of focus betwixt the central 

 and extreme ray will be as great as from a to b, while the 

 general focus will be betwixt the extremes as at B. Hence 

 the picture cannot be perfectly distinct in consequence of this 

 error, which exists not merely in theory but in practice. To 

 demonstrate this, I took a thin plate having a hole in it about 

 ith of inch in diameter, see Fig. 2 ; I then placed this little 

 hole in the centre of the object-glass as at c, Fig. 1, and, 

 having stationed a candle as at I, I tried to obtain the focal 

 imao-e as at a. I next put the little hole or aperture on the 

 object-glass at 1, and found that even then the focus was 

 considerably longer than at e . I now put the small hole to 

 the extreme part of the lens as at E, when the focus was ob- 

 served to be still farther lengthened, and, on comparing the 

 experimental result with the above theory, I found them 

 exactly to agree. 



I shall next endeavour to demonstrate, from the above facts, 

 the very great advantage the French have over us in conse- 

 quence of these imperfections in the Daguerreotype earner;!. 



