242 



OPTICAL PROJECTION 



in the lantern of an ordinary slide or diagram. As these 

 tanks frequently require to be washed clean for other experi- 

 ments, one of the best methods of constructing them is that 

 shown in fig. 128, where a piece of smooth vulcanised tubing 

 bent into a semi-circle, is squeezed between two glass plates 

 by screws through the corners, so as to make a water-tight 

 vessel easily taken to pieces for cleaning, and unaffected by 

 most chemicals. Or the plates may be mounted between 

 outer plates of metal or ebonite, which enables plain or un- 

 perforated pieces of glass to be used. Pieces of thick solid 



rubber cut to shape may also 

 be used between the plates, but 

 require more pressure and are 

 not so safe. If a little more 

 solidity is desired for the com- 

 pressed tubing, the latter may 

 be filled with fine sand after 

 being screwed up to the proper 

 tension. 



Such tanks may be of any 

 size and thickness of content, 

 and a sufficiency should be pro- 

 vided for the proposed series of 

 experiments. Or the glass plates 



may sometimes be more conveniently introduced into some 

 such wooden frame as was shown in fig. Ill, p. 223 ; and the 

 rubber tube then squeezed into place rather tightly, will 

 readily give a water-tight tank, only the glass and rubber 

 coming into contact with the fluid. 



Occasionally it will be needful to employ tanks made 

 entirely of glass, cemented together, according to the nature of 

 the solution. For aqueous solutions, those sold as zoophyte 

 troughs for microscopic use are convenient, and may be had 

 of almost any desired size ; but for spirituous solutions marine 

 glue must be replaced by other cements, such as are used in 



FIG. 128. Tank 



