ON THE KBW OBSERVATORY. 347 



but has not been particularly described, because time and opportunity have 

 scarcely permitted an examination of its qualifications as to self-correction 

 for temperature, which was always considered a principal desideratum. The 

 experience obtained this year, of its efficiency to a very great (if not com- 

 plete) extent in this respect, may perhaps be deemed a sufficient apology 

 lor the following details. 



Fig. 1, Plate XXI. represents the instrument closed and at work by day- 

 light. Fig. 2 represents it with some of the cases withdrawn. Figs. 3, 4 and 5 

 are projections (drawn to one-fourth of the real size) of the barometer and 

 compensating apparatus only. All the figures of Plate XXII. are sections, 

 &c. drawn to one-eighth of the real size. 



AA' A^, Plate XXII., are sections of the mahogany cases which constitute 

 the camera. A' has apertures, or diaphragms, about 2 inches broad, through 

 the right and left sides, 



a} is frame-work (composed of a pair of brackets, &c.) firmly secured in 

 its place by long bolts and nuts, &c. at the image end of the camera. 



B is the barometer compensated for temperature, in the manner to be de- 

 scribed presently. 



6S the surface of mercury in its tube. 



C is the first condensing lens, the frame-work and the shutter apparatus ; 

 the whole supported by brackets, and forming the object-end of the camera. 



c^ c* are grooved pieces ; between which slides, laterally and freely, 



c*, which is a plate having a rectangular aperture about 3 inches high and 

 2 inches broad. It can be made to admit light to the camera or exclude it 

 at pleasure, by being slid laterally. 



c^ is a second condensing lens. 



O is a diaphragm-plate. The aperture is about -^th of an inch broad and 

 about 3 inches high, 



D, an Argand lamp (of new construction, to be hereafter described). 



E is the usual mouth-piece, consisting of the two (usual) angular pieces, 

 and of two thin plates attached to their right sides, which form the lips. 



eS the vertical interval between the lips ; it is about 4 inches high and 

 about i^th of an inch broad. 



F is the slider-case attached to the pair of brackets at a}. 



y, its lower side, is as nearly plane as possible. (It should be of brass or 

 marble.) 



/*, a spring roller, shown only in fig. 2, Plate XXL, attached to the interior 

 side of the door of F. 



/*, a narrow aperture tlirough the central part of the door of F. 



G is the lens-tube, containing two groups of achromatic lenses by Voigt- 

 lander (of Vienna), The magnifying power is about =2 times. 



g^, apparatus, of sliding-pjate, &c., for the support and adjustment to 

 focus of G. 



H is the sliding-frame. 



h'', its door, with a narrow aperture near its right end. 



/j*, &c., three turnbuckles (sunk in h*). 



h}, &c., three springs attached to the interior side of h^, and acting upon 

 the back of the Daguerreotype plate (or upon a pair of glass plates, pressing 

 between them a piece of Talbotype paper) contained in H. 



y (fig. 4) represents the front of that plate or paper contained in H, 



/i^ A* are studs containing rollers. They are attached to a thin narrow 

 plate, and constitute, with it, a kind of carriage which travels on/*. 



A* h^, small screws for retaining cords. 



