Protochloride of palladium, which I introduced in 1859, is frequently employed when it is desired to increase the intensity of a negative without altering its thickness. This substance will augment the opacity 16 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 the metal. §6. THE PHOTOGRAPHIC ENLARGER. Two distinct arrangements are used for enlarging, _a_, for Low Powers varying from 1 to 25; and _b_, for High Powers from 50 to 700 diameters. a. _Low 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 achromatic combination of lenses, a _mirror_ of appropriate curvature to magnify the original negatives or objects. The advantages are easily enumerated, perfect coincidence 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. [Illustration: Fig. 46. The Photographic Enlarger.] The mirror is of 9 inches aperture, and 11-1/2 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 apparent, that although a minute object placed in one focus was perfectly reproduced at the other, seven times as large, yet a large one was not equally well defined in all its parts. I determined then to produce the enlarged image by passing a solar-beam 1-1/2 inch in diameter through the original lunar negative--placed in the focus nearest to the mirror--and allowing it to fall on a portion of the concave mirror, 1-1/2 inch in diameter, at one side of the vertex. Being reflected, it returns past the negative, and goes to form the magnified image at the other focus of the ellipse. In Fig. 46, _a_ is the heliostat on a stone shelf outside; _b_ a silvered glass mirror, to direct the parallel rays through _c_, the negative; _d_ is the elliptical mirror; _e_ an aperture to be partly closed by diaphragms; _f_ a rackwork movement carried by the tripod _g_; the curtain _h h′_ shuts out stray light from the interior of the observatory. The aperture _i_ is also diaphragmed, but is shown open to indicate the position of the heliostat, the shelf of which joins the outside of the building at _l_. The dotted line points out the course of the light, which coming from the sun falls on the heliostat mirror _a_, then on _b_, through _c_ to _d_, and thence returning through _e_ to the sensitive plate in the plate holder _k_.