A SILVERED GLASS TELESCOPE. 



61 



Protochloride of palladium, wliich I introduced in 1859, is frequently employed 

 when it is desired to increase the intensity of a negative without altering its thick- 

 ness. This substance will augment the opacity 16 times, withovit any tendency to 

 injure the image or produce markings. It is only at present kept out of general 

 use by the scarcity of the metal. 



§ 6. THE PHOTOGRAPHIC ENLARGER. 



Two distinct arrangements are used for enlarging, a, for Low Powers varying 

 from 1 to 25 ; and h, for High Powers frc«n 50 to 700 diameters. 



a. Loio Powers. 



The essential feature in this contrivance is an entire novelty in photographic 

 enlargement, and it is so superior to solar cameras, as they are called, that they are 

 never used in the observatory now. It consists in employing instead of an achro- 

 matic combination of lenses, a mirror of appropriate curvature to magnify the 

 original negatives or objects. The advantages are easily enumerated, perfect coinci- 

 dence of visual and chemical foci, flat field, absolute sharpness of definition. If the 

 negative is a fine one, the enlarged proofs Avill be as good as possible. 



Fig. 46. 



The Photographic Enlarger. 



The mirror is of 9 inches aperture, and 1 1 1 inches focal length. It was polished 

 on my machine to an elliptical figure of 8 feet distance between the- conjugate foci, 

 and was intended to magnify 7 times. At first the whole mirror was allowed to 

 officiate, the object being illuminated by diffused daylight. But it was soon ap- 



