810 THE POPULAR SCIENCE MONTHLY. 



finished piece of mechanism with adjusting screw, etc., is necessary ; 

 but the beginner may content himself with an arrangement of much 

 humbler pretensions. I have used a small plate of silver in which an 

 opening (about three eighths of an inch long and one twentieth of an 

 inch wide) had been cut, the width of the opening being reducible at 

 pleasure by means of a little door or shutter sliding smoothly in 

 grooves, and each opposing edge being faced with a strip of watch- 

 spring. Fitted closely into the end of the tube is a piece of cork or 

 wood having in its center a hole larger than the aperture in the slit 

 plate ; over this hole the slit plate is fastened, care being taken to 

 stop up any accidental holes or crevices in the cork (or wood, as the 

 case may be), in order to prevent the entry of extraneous light into 

 the tube. 



Primitive as this contrivance may appear, I have seen by its 

 means, when using two flint-glass prisms, the D line of sodium double, 

 and beautifully distinct. A slit, such as I have described, need not 

 cost more than one tenth of the price charged for a slit by the op- 

 tician. 



Fig. 2 shows a slit of this kind open. B is the sliding shutter, 

 and A is a piece soldered on the slit plate to bring the 

 surface up level with B. 



Concerning the observing telescope, perhaps Proc- 

 tor's remarks about a finder for an astronomical tele- 

 scope may be repeated : " It will be easy for the stu- 

 dent to construct one for himself, and will be a useful 

 Fia.2. exercise in optics." But in case the student may not 

 want to take the trouble, he will be glad to remember 

 that an ordinary pocket telescope or spy-glass may be purchased ready 

 made for a dollar or two, and by removing the erecting lenses a small 

 astronomical telescope may be produced which, as the magnifying 

 power required is small, will answer every purpose of the beginner. 



The stand may be made in a variety of styles, from the unpretend- 

 ing box on legs, with holes in the sides for the collimator and tele- 

 scope, to the highly finished tripod of the most aristocratic-looking in- 

 strument. The following is one way of making a cheap and at the 

 same time serviceable stand : Procure two disks of seasoned walnut or 

 mahogany or any other hard wood, one about a foot in diameter by 

 three eighths of an inch in thickness, and the other six inches by one 

 quarter of an inch ; also get two strips of wood about eight inches 

 long and an inch and a half wide. 



Make the larger disk into a table by screwing on three feet metal 

 hooks such as are used for hanging up clothes make excellent feet ; 

 then make a hole in the center of the large disk, and a corresponding 

 hole in one end of each of the strips. Pass a screw downward through 

 the two strips and through the hole in the disk, and let there be a 

 thumb-nut on the screw so that it may be tightened underneath. 



