Io8 THE ART OF PROJECTING. 



screen, pieces of glass of various colors may be inter- 

 posed between the lens and the bubbles, when dark or 

 black bands will be seen to take the place of those 

 colors that have been stopped by the tinted glass. 



Yellow light that is nearly monochromatic can be 

 obtained by interposing a crystal of bichromate of 

 potash. Let the crystal be a thin and quite clear one. 



Colored solutions may be used for the same purpose. 



Under the head of Spectrum Analysis .other means 

 for producing monochromatic light will be found, with 

 colored lights which are appropriate for examining 

 bubbles. 



Bubbles made of common soap-suds will not last 

 long, and various preparations have been described 

 for making persistent bubbles, some of which would 

 last three days. 



A piece of glycerine soap about the size of a marble, 

 sliced and dissolved in water at a 110 Fall., will make 

 a bubble that will last half an hour. Prof. Cooke gives 

 the following method for making a still more persistent 

 bubble : 



" Procure a quart bottle of clear glass, and some of 

 the best white castile soap (or, still better, pure palm- 

 oil soap). Cut the soap (about four ounces) into thin 

 shavings, and having put them into the bottle fill 

 this up with distilled or rain water, and shake it 

 well together. Repeat the shaking until you get a 

 saturated solution of soap. If, on standing, the solu- 

 tion settles perfectly clear, you are prepared for the 

 next step ; if not, pour off the liquid and add more 

 water to the same shavings, shaking as before. The 

 second trial will hardly fail to give you a clear solution. 

 Then add to two volumes of soap solution one volume 

 of pure concentrated glycerine. 



