PROJECTIONS. 33 



TO DETERMINE THE MAGNIFYING POWER OF A LENS, OR 

 A COMBINATION OF LENSES, IN PROJECTING. 



It will be evident, upon inspection, that the farther 

 away the screen is from the lens, the larger will be the 

 picture ; but for a given projection, the simplest way of 

 determining the magnification is to choose some object 

 of known dimensions for projection, and then to meas- 

 ure its size upon the screen. Suppose it be a lead 

 pencil having a diameter of one fourth of an inch. If 

 its image is a foot in diameter, it is evident that it is 

 magnified 4 X 12 =48 diameters. If it is three feet in 

 diameter, then it has been magnified 4 X 12 X 3 = 144 

 diameters. It will be convenient to have a scale, either 

 photographed or etched upon glass, for the purpose of 

 directly showing the magnifying power of lenses. 



A vernier made upon glass by either of the described 

 methods, will be convenient for study, and some meas- 

 urements. 



THE ANIMALCULE CAGE. 



If one would exhibit the minute forms of life to be 

 seen in water, an animalcule cage will be needed. 

 This may be made in the following way : 



Take two quite clear pieces of white glass, about four 

 inches long and one inch wide. Two other pieces of 



the same width, 

 and one inch and 

 a half long. Put 

 these two short- 



19. er pieces be- 



tween the longer ones, so as to separate them, and 

 leave a space in the middle clear through. Fasten 

 these together with japan varnish, being careful not to 

 get any of the varnish into the opening. If any should 



