PROJECTIONS. 35 



nearly as well ; the tubing may be filled with sand to 

 keep it firm. Such a tank will hold any kind of a so- 

 lution, and may be quickly taken apart and cleaned. 

 A tank which will answer for many experiments nearly 

 as well can be made by cutting a semi-circular piece out 

 of a board, of the proper size, and fastening the glass 

 sides to it with cement. What is known as marine glue 

 will be the best for this purpose, and as it is very 

 convenient to have some of this glue for making 

 and mending apparatus, because it will adhere to any 

 surface, the method of preparing it is given : Dissolve, 

 separately, equal parts of shellac and India rubber in 

 naptha, and afterwards mix the solutions thoroughly, 

 applying heat. It may be made thinner by adding 

 more naptha. It may be preserved in a tin box. In 

 order to use it, it must be heated, as well as the sur- 

 faces which are to receive it. Marine glue may be dis- 

 solved in ether, or a solution of potash. 



A METHOD FOR PROJECTING LARGE PIECES OF APPARATUS. 



Many pieces of apparatus used in illustration and 

 demonstration are much too large to be projected in the 

 ordinary way, as it is obvious that the size of the lens 

 used as condenser will be the limit to the size of the 

 object that can be shown with it. Thus, if sunlight is 

 used, the diameter of the orifice c, ^(Fig. ii), will be 

 the measure of the largest picture that can be shown at 

 once ; and if a lantern is employed, no picture larger 

 than the condenser can be projected. 



Suppose that it is desirable to show to an audience 

 a piece of apparatus much too large for ordinary pro- 

 jection, and yet too small to be plainly seen, such, for 

 instance, as the electroscope ; or the movement of a 

 pith-ball under electrical excitement j or the movement 



