102 THE ART OF PROJECTING. 



glass a little smaller than the mouth of the funnel, and 

 fasten it there with wax or putty ; break off the stem ; 

 put the prism under water, and when it is filled stop 

 it with a cork : it is now ready for work. 



Next cut out a semicircular slit from a piece of paste- 

 board. It need not be more than the one sixteenth of 

 an inch broad; and it may be an inch across, as repre- 

 sented at A, Fig. 78. 



Fig. 78. 



Place the condenser in front of the aperture, and 

 hold the prism near the focus, and then a little distance 

 back of it may be placed the pasteboard with the slit s. 

 The semicircular diverging beam is refracted, and 

 suffers dispersion ; which gives a very good bow, but 

 with the red innermost, like the secondary bow. 



Still another way is possible: If a beam of light 

 fall upon a cylindrical reflector, like a glass rod, or 

 even a tin tube, like the handle to a tin dipper, the 

 light is reflected from it in a large nearly complete 

 circle. Place such a reflector at s above, in place of 

 the pasteboard slit, and then with the prism a bow will 



