A SILVERED GEARS T E T> E S C O P E . 



51 



Protochloride of palladium, which T introduced in 1859, is frequently employed 

 when it is desired to increase the intensity of a nejj;ative without alterinjij its thick- 

 ness. This suhstance will augment the opacity 1(5 times, without any tendency to 

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

 use by the scarcity of tlie metal. 



§0. THE riroTO(;i;Ariiir enlarger. 



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

 from 1 to 25 ; and b, for High Powers from 50 to 700 diameters. 



a. Lov) 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 will be as good as possible. 



Pig. 46. 



The Photographic Enlarger. 



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

 on my machine to an elliptical figur(> of >^ feet distance between the conjugate foci, 

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

 officiate, the ol)jcct being illuminated by diffused daylight. But it was soon a]^^)- 



