3S Description of u Cvmetariurn . 



afford amusement to SQme of your readers, T have taken the 

 liberty to trouble you with a description of it. 



The first part of my instrqment that I shall describe con- 

 sists of a mahogany board, l-l inches in length and lo in 

 breadth, fixed to a stand in a perpendicular direction, in the 

 following manner : 



A brass pin, sufficiently strong to bear the weight of the 

 instrument, is screwed into the side of a stand of a tele- 

 scope ; the other end of the pin is tapped, and has a button 

 screw that turns upon it. This pin goes through a hole 

 near the bottom of the board, and by means of the button 

 screw and brass washers the hoard is made fast to the stand. 

 Near the top of the board a hole 6 inches wide and 3 inches 

 high is cut through it ; the top of the hole being 3 inches 

 from the top of the board. 



2dly. A double convex lens, 4 inches in diameter and 24 

 inches focal distance, set in a piece of mahogany 6 inches 

 square, is mounted in a square frame 6 inches within. The 

 lens turns round within this frame upon two pins, like a 

 looking-glass that is made to stand upon a table. This 

 frame is made fast to the board with screws around the hole 

 above mentioned, in such a manner that one part of the lens 

 may be on one side of the board, and the other part on the 

 other side. That side of the board on which the frame is 

 fixed, I shall, for the sake of distinction, call the back part 

 of the instrument. 



About 2i inches below the centre of this lens, another 

 lens of half an inch in diameter, and 36 inches focal distance, 

 is placed on the back part of the instrument, against a hole 

 made through the mahogany board. As this lens requires 

 adjustment, it is set in a small piece of wood that slides in a 

 dove-tail frame fixed to the back part of the instrument ; 

 and a lens of l^ inch diameter, and 40 inches focal distance, 

 is fixed in a hole near the bottom of the board. 



Directions fur using the Instrument. 

 Having set the instrument upf)n a table, with its back 

 turned towards a white waH, at the distance of 2J inches, 

 turn that part of the great leni that is behind the instrument 



until 



