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THE ART OF PROJECTING. 



very small beam be used, the room will need to be 

 quite dark. When it is desirable to have a large and 

 brilliant spectrum, the light may be sent through a 

 condenser, with a focus one or two feet long, and using 



Fig, 82, 



a diaphragm near the focus to cut off the marginal 

 rays. This will permit much more light to be properly 



refracted for a good 

 spectrum. At a 

 distance of twenty- 

 five feet, a common 

 triangular prism 

 will give a spec- 

 Fig. 83, trum about five 



feet long; but it may be indefinitely lengthened by 

 inclining the screen to it, as shown above, and it will 

 usually be quite bright when made ten or fifteen {qq{ 

 long ; if the room be otherwise, quite dark. 



The dispersive power of different substances may be 

 shown by making a V trough of glass, with an included 

 angle of sixty degrees. Water, alcohol, ether, spirits 

 of turpentine, etc., may be put in it, and the beam sent 

 through them. In this case the spectrum will appear 

 overhead upon the ceiling. If the glass trough have 

 three or four tight partitions in it, all the substances 

 may be used at once, and thus their refractive powers 

 compared. 



