OF ARTS AND SCIENCES. 329 



screwed to this disk, rises from it ; the two together giving a height of 

 twelve inches from the top of the platform. The upper portion of 

 this tripod is a flat ring, four inches in diameter. To this is screwed 

 an oblong square of wood, seven inches in length by four in breadth 

 and one and a half in thickness, which we may call the hed. In the 

 middle of this hed is an angular groove, to which a lining of thick 

 tinfoil is accurately pasted or glued. In this angular groove slides a 

 piece of pine of the same length, with a rounded groove on its upper 

 surface, carrying the tube, and which may be called the cradle. Its 

 width is an inch and a half, and its upper surface one inch above that 

 of the piece on which it rests. The stage is five inches broad by four 

 in height and one fourth of an inch thick, secured firmly to the end of 

 the hed. A round hole in its centre, three fourths of an inch in diame- 

 ter, is centred with the end of the tube, and bevelled half an inch out- 

 ward on the side toward the light, to allow greater obliquity of illumi- 

 nation. The shelf supporting the lamp is five inches square, and is 

 supported by two stout pins received in two holes passing through the 

 lower part of the stage into the end of the hed^ so as to be easily re- 

 moved for packing. A strong wire, two inches and a half long, is 

 soldered to the middle of each side of the tube at right angles. By 

 these wires the tube is slid backward and forward in the cradle., form- 

 ing the coarse adjustment. A brass spiral spring is fastened in the an- 

 terior end of the groove in the hed. The short arm of the lever, (the 

 long arm of which, seen in the woodcut, is moved by the screw 

 below,) passing through a hole in the middle of the hed into a notch 

 in the under part of the cradle., presses the cradle against the spiral 

 spring. The whole length of the lever is twelve inches, that of the 

 short arm three fourths of an inch, giving a ratio of one to fifteen in 

 the two arms. The screw, which plays in a brass nut, has sixteen 

 threads to the inch, so that one revolution moves the cradle and tube 

 one two-hundred-and-fortieth of an inch. This is ihejine adjustment. 

 The head of the screw, two inches in diameter, is not milled, but scal- 

 loped, so that the forefinger lies easily in the hollows, and turns it 

 either way. The stage being an inch wider than the bed, gives room 

 on each side for the attachment of a flat brass spring, serving to hold 

 the object-plate against the stage, at the same time permitting it to be 

 moved freely in every direction. The object-plate itself is of brass, 

 eight inches long by an inch and three quarters wide, with a hole three 

 fourths of an inch in diameter in its centre, and below the hole a ledge 

 VOL. II. 42 



