CHAP, vii.] PHOTOGRAPHY ON PAPER AND ON GLASS. 



305 



effects by first placing in the frame a ground glass, on the surface 

 of which the image can be seen. If this image be not clear, 

 the ground surface of the glass is not in focus ; and the defect must 

 be corrected by moving the sliding sides of the camera, either length- 

 ening or diminishing the distance till the exact focus is found ; this 

 is called " focussing the image," a similar operation to that which we 

 have described for the lenses of telescopes and microscopes. 



The clearness of the image depends on the quality of the object- 

 glass, which should be achromatic, and without spherical aberration. 



Figs. 220 and 221 give sections of two different forms of object- 

 glasses, some simple, others compound. The simple object-glass 

 often requires a diaphragm in front of it having a small opening. 





FIG. 219. Country photographic apparatus, bellows shape. 



The quantity of light passing through a narrow opening being limited, 

 this object-glass sometimes requires a prolonged exposure. It is 

 used more especially for views, landscapes, &c. 



The object-glass with a combination of lenses (Fig. 221), and with 

 the diaphragm placed between them, permits the entrance of a larger 

 quantity of light ; it is employed, in preference, for portraits, because 

 the exposure required is not so long. 



In daguerreotypes the image was reversed on the plate, so that 

 the right side appeared at the left, and vice versd. To obtain a direct 

 image, either a total reflection prism, or a mirror inclined at 45, was 

 adjusted to the object-glass. This precaution is not required in 

 photography on glass, because it is the negative proof which is 

 inverted and symmetrical, and by turning it to obtain the positive 

 proof, the latter is found to be in the normal position. 



x 



